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Habitats and Ecosystems: Seagrasses

• Short, F.T. et al.  Extinction risk assessment of the world's seagrass species.  Biological Conservation 144(7): 1961-1971, 2011.  
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  Seagrasses, a functional group of marine flowering plants rooted in the world's coastal oceans, support marine food webs and provide essential habitat for many coastal species, playing a critical role in the equilibrium of coastal ecosystems and human livelihoods. For the first time, the probability of extinction is determined for the world's seagrass species under the Categories and Criteria of the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species. Several studies have indicated that seagrass habitat is declining worldwide. Our focus is to determine the risk of extinction for individual seagrass species, a 4-year process involving seagrass experts internationally, compilation of data on species' status, populations, and distribution, and review of the biology and ecology of each of the world's seagrass species. Ten seagrass species are at elevated risk of extinction (14% of all seagrass species), with three species qualifying as Endangered. Seagrass species loss and degradation of seagrass biodiversity will have serious repercussions for marine biodiversity and the human populations that depend upon the resources and ecosystem services that seagrasses provide.

• Matheson, F.E., Lundquist, C.J., Gemmill, C.E.C., and Pilditch, C.A.  New Zealand seagrass – More threatened than IUCN review indicates.  Biological Conservation 144(12): 2749-2750, 2011.
  
• Franssen, S.U., Gu, J., Bergmann, N., Winters, G., Klostermeier, U.C., Rosenstiel, P., Bornberg-Bauer, E., and Reusch, T.B.H.  Transcriptomic resilience to global warming in the seagrass Zostera marina, a marine foundation species.  Proceedings of the National Academy of Sciences [USA] 108(48): 19276-19281, 2011.   
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  Large-scale transcription profiling via direct cDNA sequencing provides important insights as to how foundation species cope with increasing climatic extremes predicted under global warming. Species distributed along a thermal cline, such as the ecologically important seagrass  Zostera marina, provide an opportunity to assess temperature effects on gene expression as a function of their long-term adaptation to heat stress. We exposed a southern and northern European population of Zostera marina from contrasting thermal environments to a realistic heat wave in a common-stress garden. In a fully crossed experiment, eight cDNA libraries, each comprising ~125 000 reads, were obtained during and after a simulated heat wave, along with nonstressed control treatments. Although gene-expression patterns during stress were similar in both populations and were dominated by classical heat-shock proteins, transcription profiles diverged after the heat wave. Gene-expression patterns in southern genotypes returned to control values immediately, but genotypes from the northern site failed to recover and revealed the induction of genes involved in protein degradation, indicating failed metabolic compensation to high sea-surface temperature. We conclude that the return of gene-expression patterns during recovery provides critical information on thermal adaptation in aquatic habitats under climatic stress. As a unifying concept for ecological genomics, we propose transcriptomic resilience, analogous to ecological resilience, as an important measure to predict the tolerance of individuals and hence the fate of local populations in the face of global warming.

• Costello, C.T. and Kenworthy, W.J.  Twelve-year mapping and change analysis of eelgrass (Zostera marina) areal abundance in Massachusetts (USA) identifies statewide declines.  Estuaries and Coasts 34(2): 232-242, 2011.
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  In 1994, 1995, and 1996, seagrasses in 46 of the 89 coastal embayments and portions of seven open-water near-shore areas in Massachusetts were mapped with a combination of aerial photography, digital imagery, and ground truth verification. In the open-water areas, 9,477.31 ha of seagrass were identified, slightly more than twice the 4,846.2 ha detected in the 46 coastal embayments. A subset of the 46 embayments, including all regions of the state were remapped in 2000, 2001, and 2002 and again in 2006 and 2007. We detected a wide range of changes from increases as high as 29% y^-1 in Boston Harbor to declines as large as -33% y^-1 in Salem Harbor. One embayment, Waquoit Bay, lost all of its seagrass during the mapping period. For the 12-year change analysis representing all geographic regions of the state, only three embayments exhibited increases in seagrass coverage while 30 of the original 46 embayments showed some indication of decline. For the decadal period, rates of decline in the individual embayments ranged from -0.06% y^-1 to as high as -14.81% y^-1. The median rate of decline by region ranged from -2.21% y^-1 to -3.51% y^-1 and was slightly less than the recently reported global rate of decline for seagrasses (-3.7% y^-1). Accounting for the gains in three of the embayments, 755.16 ha (20.6%) of seagrass area originally detected was lost during the mapping interval. The results affirm that previously reported losses in a few embayments were symptomatic of more widespread seagrass declines in Massachusetts. State and Federal programs designed to improve environmental quality for conservation and restoration of seagrasses in Massachusetts should continue to be a priority for coastal managers.

• Garcias-Bonet, N., Sherman, T.D., Duarte, C.M., and Marbà, N.  Distribution and pathogenicity of the protist Labyrinthula sp. in western Mediterranean seagrass meadows.  Estuaries and Coasts 34(6): 1161-1168, 2011.
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  The presence of the pathogenic protist Labyrinthula sp., the causative agent of seagrass wasting disease and mass mortality events, was assessed in 18 seagrass meadows in the Balearic region (western Mediterranean). This protist was found in 70% of seagrass meadows investigated and in all seagrass species present in the region (i.e., Posidonia oceanica, Cymodocea nodosa, and Zostera noltii). Labyrinthula spp. cultures isolated from seven P. oceanica and one Thalassia testudinum meadows were used as inocula to perform cross-infection experiments in order to test seagrass vulnerability to Labyrinthula spp. infection. These isolates produced lesions on P. oceanica and other seagrass species (Zostera marina, Z. noltii, and C. nodosa). P. oceanica and Z. noltii, both species autochthonous to the Mediterranean Sea, were the seagrasses most vulnerable to infection by the tested isolates. One of the P. oceanica isolates of Labyrinthula sp. also infected the Atlantic seagrass Z. marina, and all of the Mediterranean seagrasses were infected by Labyrinthula sp. isolated from the T. testudinum, native to the Caribbean and Gulf of Mexico. This work confirms that Labyrinthula sp. is commonly found on seagrasses of the Mediterranean Sea and demonstrates that Labyrinthula sp. can infect seagrasses in different genera, in contrast to previous studies where Labyrinthula sp. was considered to be genus-specific. This finding points out the broadly pathogenic nature of some Labyrinthula sp. isolates. Finally, this work identifies Labyrinthula sp. as a possible detrimental agent for P. oceanica.

• Wall, C.C., Peterson, B.J., and Gobler, C.J.  The growth of estuarine resources (Zostera marina, Mercenaria mercenaria, Crassostrea virginica, Argopecten irradians, Cyprinodon variegatus) in response to nutrient loading and enhanced suspension feeding by adult shellfish.  Estuaries and Coasts 34(6): 1262-1277, 2011.
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  While many coastal ecosystems previously supported high densities of seagrass and abundant bivalves, the impacts of overfishing, eutrophication, harmful algal blooms, and habitat loss have collectively contributed to the decline of these important resources. Despite improvements in wastewater treatment in some watersheds and subsequent reduced nutrient loading to neighboring estuaries, seagrass and bivalve populations in these locations have generally not recovered. We performed three mesocosm experiments to simultaneously examine the contrasting effects of nutrient loading and historic suspension-feeding bivalve densities on the growth of eelgrass (Zostera marina), juvenile bivalves (northern quahogs, Mercenaria mercenaria; eastern oysters, Crassostrea virginica; and bay scallops, Argopecten irradians), and juvenile planktivorous fish (sheepshead minnow, Cyprinodon variegatus). High nutrient loading rates led to significantly higher phytoplankton (chlorophyll a) levels in all experiments, significantly increased growth of juvenile bivalves relative to controls with lower nutrient loading rates in two experiments, and significantly reduced the growth of eelgrass in one experiment. The filtration provided by adult suspension feeders (M. mercenaria and C. virginica) significantly decreased phytoplankton levels in all experiments, significantly increased light penetration and the growth of eelgrass in one experiment, and significantly decreased the growth of juvenile bivalves and fish in two experiments, all relative to controls with no filtration from adult suspension feeders. These results demonstrate that an appropriate level of nutrient loading can have a positive effect on some estuarine resources and that bivalve filtration can mediate the effects of nutrient loading to the benefit or detriment of different estuarine resources. Future ecosystem-based approaches will need to simultaneously account for anthropogenic nutrient loading and bivalve restoration to successfully manage estuarine resources.

• Nakamura, Y.  Patterns in fish response to seagrass bed loss at the southern Ryukyu Islands, Japan.  Marine Biology 157(11): 2397-2406, 2010.
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  An extensive seagrass bed on a fringing coral reef at Amitori Bay (southern Ryukyu Islands) disappeared completely in 2009 after a typhoon. Seagrass bed loss had a significant negative influence on not only seagrass bed residents but also commercially important coral reef fishes that utilize seagrass beds as nurseries or feeding grounds. With seagrass bed loss, mean species' richness and densities of overall seagrass bed fishes per transect decreased by more than 75 and 85%, respectively. Most of the affected fishes were benthivores, piscivores, detritivores, and herbivores. Of 21 dominant species, 13 disappeared completely and 4 showed severe reductions in densities following seagrass bed loss, whereas the densities of 4 bottom-dwelling gobies did not change significantly. Thus, this study demonstrated that most seagrass bed fishes lack the ability to adapt to seagrass habitat loss, suggesting that increasing global seagrass loss will cause serious reductions in seagrass-associated fishes and fishery resources.

• Holmer, M., Wirachwong, P., and Thomsen, M.S.  Negative effects of stress-resistant drift algae and high temperature on a small ephemeral seagrass species.  Marine Biology 158(2): 297-309, 2011.
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  Seagrasses are threatened by multiple anthropogenic stressors, such as accumulating drift algae and increasing temperatures (associated with eutrophication and global warming, respectively). However, few seagrass experiments have examined whether exposure to multiple stressors causes antagonistic, additive, or synergistic effects, and this has limited our ability to predict the future health status of seagrass beds. We conducted a laboratory experiment to test whether abundance of Gracilaria comosa (3 levels; 0, 1.2, and 3.4 kg WW m^-2), an algae that is resistant to wide environmental fluctuations (e.g. light, temperature, salinity, and oxygen levels), has negative effects on the small ephemeral seagrass, Halophila ovalis and whether the effects are exacerbated by high temperature (3 levels; 20, 25, and 30 ºC). We found an additive negative effect of the two stressors when tested simultaneously on 14 seagrass performance measures, with most data variability explained by the drift algae. For the individual plant performance measures (above- and below-ground growth and mortality, leaf area, internode distance, and root length and root volume), we found 5 additive effects, 4 synergistic effects, and 5 effects that were significant only for drift algae. We also documented a significant additive effect of drift algae and temperature on dissolved porewater sulphide (DS). A follow-up correlation analysis between DS and the 14 plant performance measures revealed significant or near-significant linear correlations on 9 of these responses (above- and below-ground growth, leaf area and weight, leaf mortality, and internode distance). In summary, we showed (a) that a stress-resistant drift algae can have strong negative effects on a small ephemeral seagrass, (b) this negative effect can increase both additively and synergistically with increasing temperature depending on performance measure, and (c) the negative effects may be mediated by a build-up of porewater DS. An implication of our findings is that resource managers aiming to preserve healthy seagrass beds in an almost certain future warmer world should increase efforts to keep drift algae populations low.

• Pillay, D., Branch, G.M., Griffiths, C.L., Williams, C., and Prinsloo, A.  Ecosystem change in a South African marine reserve (1960-2009): role of seagrass loss and anthropogenic disturbance.  Marine Ecology Progress Series 415: 35-48, 2010.
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  Seagrass ecosystems are disappearing throughout the world, raising several concerns for ecosystem functioning and biodiversity. In the present study we document changes in intertidal sandflat ecosystems over the last 5 decades in Langebaan Lagoon, a marine reserve on the west coast of South Africa, following large-scale losses of seagrass Zostera capensis. Aerial photography and GIS revealed a 38% loss of seagrass at sites in Langebaan Lagoon between 1960 and 2007. Long-term monitoring of one particular site, Klein Oesterwal, indicated a major shift in invertebrate communities between 1983, when Z. capensis was abundant, and 2009, when Z. capensis had virtually disappeared. Abundance and species richness of invertebrates declined significantly over this period. Seagrass-associated species such as the limpets Siphonaria compressa and Fissurella mutabilis, and the starfish Parvulastra exigua, declined almost to extinction locally. Generalists such as the gastropod Assiminea globulus also declined in abundance. However, sandflat species, particularly burrowers, increased in abundance and vertical range. Wading birds also appeared to be affected by changes in seagrass cover. The terek sandpiper, which depends on Z. capensis for feeding, showed 3 local population crashes, each corresponding to periods of seagrass collapse. Abundance of wading birds that were not dependant on this seagrass was more stable. The loss of seagrasses in the system, together with anthropogenic disturbance such as bait harvesting and trampling, may have had severe impacts on invertebrates, the most significant being the virtual extinction locally of the critically endangered limpet S. compressa. Cascading effects on wading birds, possibly through changes in availability of invertebrate prey, were also evident.

• Rasheed, M.A. and Unsworth, R.K.F.  Long-term climate-associated dynamics of a tropical seagrass meadow: implications for the future.  Marine Ecology Progress Series 422: 93-103, 2011.
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  The long-term changes of tropical intertidal seagrass, mainly Halodule uninervis and Halophila ovalis meadows and their relationship to climate are poorly documented. Developing a greater understanding of the effects of climate on seagrass meadows is critical for estimating the effects of future climate change scenarios. Here we document the temporal dynamics of coastal intertidal seagrass in tropical northeast Australia over 16 yr of detailed monitoring. This study is the first to directly relate such change to long-term climate variability in the Indo-Pacific region and southern hemisphere. Regression modelling was used to relate seagrass biomass and meadow area measurements to climate data. The aboveground biomass and area of the meadow were correlated with the interacting factors of air temperature, precipitation, daytime tidal exposure and freshwater runoff from nearby rivers. Elevated temperature and reduced flow from rivers were significantly correlated (R² = 0.6, p < 0.001) with periods of lower seagrass biomass. Results of this study have important implications for the long-term viability of seagrasses with regard to climate change scenarios. Modelling of our findings indicates that future higher temperatures could be detrimental to Indo-Pacific intertidal, coastal and estuarine seagrass meadows.

• Vaudrey, J.M.P., Kremer, J.N., Branco, B.F., and Short, F.T.  Eelgrass recovery after nutrient enrichment reversal.  Aquatic Botany 93(4): 237-243, 2010.
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  Mumford Cove, a 48 ha Connecticut embayment on Long Island Sound, has a history of excessive nutrient inputs and corresponding eutrophic conditions with concomitant eelgrass (Zostera marina L) loss. From 1945 to 1987, a municipal wastewater treatment facility discharged into the cove. In 1987, when the wastewater outfall was diverted to another location, the cove supported a near monoculture of the green algae Ulva lactuca L., covering 74% of the bottom. By 1988, macroalgal areal cover in Mumford Cove had declined to 9%. When we first sampled the cove in 1992, Z. marina and Ruppia maritima L were present. By 1999, areal distribution of Z. marina and R. maritima had expanded to cover a third of the bottom. Periodic summertime surveys from 2002 through 2004 indicated that Z. marina was present in approximately half of the cove; R. maritima was sparse. Fifteen years after termination of nutrient enrichment, this cove had recovered from 40 years of point source anthropogenic nutrient input, returning from an Ulva-dominated to a Zostera-dominated state.

• van der Heide, T., van Nes, E.H., van Katwijk, M.M., Olff, H., and Smolders, A.J.P.  Positive feedbacks in seagrass ecosystems – evidence from large-scale empirical data.  PLoS ONE 6(1): art. e16504, 2011.
  Open Access >>
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  Positive feedbacks cause a nonlinear response of ecosystems to environmental change and may even cause bistability. Even though the importance of feedback mechanisms has been demonstrated for many types of ecosystems, their identification and quantification is still difficult. Here, we investigated whether positive feedbacks between seagrasses and light conditions are likely in seagrass ecosystems dominated by the temperate seagrass Zostera marina. We applied a combination of multiple linear regression and structural equation modeling (SEM) on a dataset containing 83 sites scattered across Western Europe. Results confirmed that a positive feedback between sediment conditions, light conditions and seagrass density is likely to exist in seagrass ecosystems. This feedback indicated that seagrasses are able to trap and stabilize suspended sediments, which in turn improves water clarity and seagrass growth conditions. Furthermore, our analyses demonstrated that effects of eutrophication on light conditions, as indicated by surface water total nitrogen, were on average at least as important as sediment conditions. This suggests that in general, eutrophication might be the most important factor controlling seagrasses in sheltered estuaries, while the seagrass-sediment-light feedback is a dominant mechanism in more exposed areas. Our study demonstrates the potentials of SEM to identify and quantify positive feedbacks mechanisms for ecosystems and other complex systems.

• Irving, A.D., Connell, S.D., and Russell, B.D.  Restoring coastal plants to improve global carbon storage: Reaping what we sow.  PLoS ONE 6(3): art. e18311, 2011.
  Open Access >>
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  Long-term carbon capture and storage (CCS) is currently considered a viable strategy for mitigating rising levels of atmospheric CO2 and associated impacts of global climate change. Until recently, the significant below-ground CCS capacity of coastal vegetation such as seagrasses, salt marshes, and mangroves has largely gone unrecognized in models of global carbon transfer. However, this reservoir of natural, free, and sustainable carbon storage potential is increasingly jeopardized by alarming trends in coastal habitat loss, totalling 30-50% of global abundance over the last century alone. Human intervention to restore lost habitats is a potentially powerful solution to improve natural rates of global CCS, but data suggest this approach is unlikely to substantially improve long-term CCS unless current restoration efforts are increased to an industrial scale. Failure to do so raises the question of whether resources currently used for expensive and time-consuming restoration projects would be more wisely invested in arresting further habitat loss and encouraging natural recovery.

• Coll, M., Schmidt, A., Romanuk, T., and Lotze, H.K.  Food-web structure of seagrass communities across different spatial scales and human impacts.  PLoS ONE 6(7): art. e22591, 2011.
  Open Access >>
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  Seagrass beds provide important habitat for a wide range of marine species but are threatened by multiple human impacts in coastal waters. Although seagrass communities have been well-studied in the field, a quantification of their food-web structure and functioning, and how these change across space and human impacts has been lacking. Motivated by extensive field surveys and literature information, we analyzed the structural features of food webs associated with Zostera marina across 16 study sites in 3 provinces in Atlantic Canada. Our goals were to (i) quantify differences in food-web structure across local and regional scales and human impacts, (ii) assess the robustness of seagrass webs to simulated species loss, and (iii) compare food-web structure in temperate Atlantic seagrass beds with those of other aquatic ecosystems. We constructed individual food webs for each study site and cumulative webs for each province and the entire region based on presence/absence of species, and calculated 16 structural properties for each web. Our results indicate that food-web structure was similar among low impact sites across regions. With increasing human impacts associated with eutrophication, however, food-web structure show evidence of degradation as indicated by fewer trophic groups, lower maximum trophic level of the highest top predator, fewer trophic links connecting top to basal species, higher fractions of herbivores and intermediate consumers, and higher number of prey per species. These structural changes translate into functional changes with impacted sites being less robust to simulated species loss. Temperate Atlantic seagrass webs are similar to a tropical seagrass web, yet differed from other aquatic webs, suggesting consistent food-web characteristics across seagrass ecosystems in different regions. Our study illustrates that food-web structure and functioning of seagrass habitats change with human impacts and that the spatial scale of food-web analysis is critical for determining results.

• Fonseca, M.S.  Addy revisited: What has changed with seagrass restoration in 64 years?  Ecological Restoration 29(1-2): 73-81, 2011.
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  A brief appraisal of the present state of seagrass restoration in the context of the 64-year-old seminal publication by C.E. Addy reveals that early observations were prescient and have remained the basis for our collective attempts to conduct open system seagrass restoration. Our ability to ensure restoration success remains limited. A flawed philosophical framework for choosing restoration, frequently exacerbated by management inexperience and failure to apply known standards for site selection, continues to plague the process. Moreover, seagrass restoration has become an on-demand attempt to overcome hysteresis and shift a habitat from one stable state (unvegetated) to what is arguably a more complex stable state (vegetated) by artificial colonization methods. These methods are frequently overwhelmed by natural processes that ordinarily rely on orders of magnitude more propagules and years of recruitment classes. As a result, the expectations for successful seagrass restoration, like most wild community restoration projects, are often unrealistic and improperly held to an even higher standard than agricultural crops. Limited attention to project monitoring, lack of practical recovery metrics, and, in some cases, limited scientific knowledge of seagrass (e.g., population ecology, genomics, and landscape dynamics) limit our capability to generate quantitative guidance and realistic expectations. Improving the probability of successful restoration depends not so much on overcoming technical transplanting issues, but on avoiding injudicious reliance on restoration to solve higher-level resource management issues. Overall, conservation of seagrass remains a more reasonable alternative than restoration, and Addy is revealed as a keen observer of the field.

• van Katwijk, M.M., van der Welle, M.E.W., Lucassen, E.C.H.E., Vonk, J.A., Christianen, M.J.A., Kiswara, W., Inayat al Hakim, I., Arifin, A., Bouma, T.J., Roelofs, J.G.M., and Lamers, L.P.M.  Early warning indicators for river nutrient and sediment loads in tropical seagrass beds: A benchmark from a near-pristine archipelago in Indonesia.  Marine Pollution Bulletin 62(7): 1512-1520, 2011.
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  In remote, tropical areas human influences increase, potentially threatening pristine seagrass systems. We aim (i) to provide a bench-mark for a near-pristine seagrass system in an archipelago in East Kalimantan, by quantifying a large spectrum of abiotic and biotic properties in seagrass meadows and (ii) to identify early warning indicators for river sediment and nutrient loading, by comparing the seagrass meadow properties over a gradient with varying river influence. Abiotic properties of water column, pore water and sediment were less suitable indicators for increased sediment and nutrient loading than seagrass properties. Seagrass meadows strongly responded to higher sediment and nutrient loads and proximity to the coast by decreasing seagrass cover, standing stock, number of seagrass species, changing species composition and shifts in tissue contents. Our study confirms that nutrient loads are more important than water nutrient concentrations. We identify seagrass system variables that are suitable indicators for sediment and nutrient loading, also in rapid survey scenarios with once-only measurements.

• Gao, Y., Fang, J., Zhang, J., Ren, L., Mao, Y., Li, B., Zhang, M., Liu, D., and Du, M.  The impact of the herbicide atrazine on growth and photosynthesis of seagrass, Zostera marina (L.), seedlings.  Marine Pollution Bulletin 62(8): 1628-1631, 2011.
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  The impact of the widely used herbicide atrazine on seedling growth and photosynthesis of eelgrass was determined. The long-term impact of the herbicide atrazine (1, 10 and 100 µg/L) on growth of eelgrass Zostera marina (L.) seedlings, maintained in outdoor aquaria, was monitored over 4 weeks. Exposure to 10 µg/L atrazine resulted in significantly lower plant fresh weight and total chlorophyll concentration and up to 86.67% mortality at the 100 µg/L concentration. Short-term photosynthetic stress on eelgrass seedlings was determined and compared with adult eelgrass using chlorophyll fluorescence. The effective quantum yield in eelgrass seedlings was significantly depressed at all atrazine concentrations (2, 4, 8, 16, 32 and 64 µg/L) even within 2 h and remained at a lower level than for adult plants for each concentration. These results indicate that atrazine presents a potential threat to seagrass seedling functioning and that the impact is much higher than for adult plants.

• Taylor, H.A. and Rasheed, M.A.  Impacts of a fuel oil spill on seagrass meadows in a subtropical port, Gladstone, Australia – The value of long-term marine habitat monitoring in high risk areas.  Marine Pollution Bulletin 63(5-12): 431-437, 2011.
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  We used an established seagrass monitoring programme to examine the short and longer-term impacts of an oil spill event on intertidal seagrass meadows. Results for potentially impacted seagrass areas were compared with existing monitoring data and with control seagrass meadows located outside of the oil spill area. Seagrass meadows were not significantly affected by the oil spill. Declines in seagrass biomass and area 1 month post-spill were consistent between control and impact meadows. Eight months post-spill, seagrass density and area increased to be within historical ranges. The declines in seagrass meadows were likely attributable to natural seasonal variation and a combination of climatic and anthropogenic impacts. The lack of impact from the oil spill was due to several mitigating factors rather than a lack of toxic effects to seagrasses. The study demonstrates the value of long-term monitoring of critical habitats in high risk areas to effectively assess impacts.

• Höffle, H., Thomsen, M.S., and Holmer, M.  High mortality of Zostera marina under high temperature regimes but minor effects of the invasive macroalgae Gracilaria vermiculophylla.  Estuarine, Coastal and Shelf Science 92(1): 35-46, 2011.
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  The present study tested for density-dependent effects of the invasive drift macroalgae Gracilaria vermiculophylla (Ohmi) Papenfuss on growth and survival of the native eelgrass, Zostera marina L., under different temperature levels. Three weeks laboratory experiments were conducted in Odense, Denmark, combining three algae densities (control, low 1.9 kg WWm^-2, high 4.5 kg WWm^-2) with typical Danish summer temperatures (18°C) and elevated temperatures (21°C and 27°C). There was a significant effect of temperature on shoot survival with on average 68% mortality in the high temperature treatment but almost no mortality at the two lower temperatures. The higher mortality was probably caused by high sulphide levels in the sediment pore water (0.6 mmoll^-1 at 18°C compared to 3.7 mmoll^-1 at 27°C). Above-ground growth of the surviving shoots was also significantly affected by temperature, with leaf elongation rates being negatively affected, while the leaf plastochrone interval increased. Relative growth rate was significantly higher at 21°C than at 18°C or 27°C, whereas rhizome elongation was significantly lowest at 27°C. Elemental sulphur content in the plant tissues increased significantly with temperature and was up to 34 times higher (S₀ in rhizomes) at 27°C compared to the lower temperatures. In contrast to the temperature effects, cover by G. vermiculophylla did not cause significant effects on any seagrass responses. However, there was a (non-significant) negative effect of algal cover at the highest temperature, where the seagrass is already stressed. The latter results suggest that more studies should test for interaction effects between temperature and other anthropogenic stressors given that temperature is predicted to increase in the near future.

• Ooi, J.L.S., Kendrick, G.A., Van Niel, K.P., and Affendi, Y.A.  Knowledge gaps in tropical Southeast Asian seagrass systems.  Estuarine, Coastal and Shelf Science 92(1): 118-131, 2011.
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  Seagrasses are habitats with significant ecological and economic functions but we have limited knowledge of seagrasses in Southeast Asia, the hypothesized centre-of-origin for tropical seagrasses. There have been only 62 ISI-cited publications on the seagrasses of Southeast Asia in the last three decades and most work has been in few sites such as Northwest Luzon in the Philippines and South Sulawesi in Indonesia. Our understanding of the processes driving spatial and temporal distributions of seagrass species here has focussed primarily on backreef and estuarine seagrass meadows, with little work on forereef systems. We used Pulau Tinggi, an island off the southeast coast of Peninsular Malaysia, as an example of a subtidal forereef system. It is characterized by a community of small and fast growing species such as Halophila ovalis (mean shoot density 1454.6 ± 145.1 m^-2) and Halodule uninervis (mean shoot density 861.7 ± 372.0 m^-2) growing in relatively low light conditions (mean PAR 162.1 ± 35.0 μmol m^-2 s^-1 at 10m depth to 405.8 ± 99.0 μmol m^-2 s^-1 at 3 m water depth) on sediment with low carbonate (mean 9.24 ±1.74 percentage dry weight), organic matter (mean 2.56 ± 0.35 percentage dry weight) and silt-clay content (mean 2.28 ± 2.43 percentage dry weight). The literature reveals that there is a range of drivers operating in Southeast Asian seagrass systems and we suggest that this is because there are various types of seagrass habitats in this region, i.e. backreef, forereef and estuary, each of which has site characteristics and ecological drivers unique to it. Based on our case study of Pulau Tinggi, we suggest that seagrasses in forereef systems are more widespread in Southeast Asia than is reflected in the literature and that they are likely to be driven by recurring disturbance events such as monsoons, sediment burial and herbivory.

• Nordlund, L., Erlandsson, J., de la Torre-Castro, M., and Jiddawi, N.  Changes in an East African social-ecological seagrass system: invertebrate harvesting affecting species composition and local livelihood.  Aquatic Living Resources 23(4): 399-416, 2010.
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  Seagrass meadows support high biodiversity and are important for invertebrate harvesting activities in developing countries. The aim of this study was to estimate the social and ecological effects of invertebrate harvesting, i.e. how this exploitation may affect/has affected seagrass variables (biomass, shoot density and canopy height), macrofaunal community structure, the use and importance of these resources for the livelihood of local people over time. A multi-disciplinary approach was used, including interviews with harvesters, observations of the number/activities of invertebrate harvesters, and a biological field study in Zanzibar, Tanzania. The study showed that women/children harvest invertebrates, and they prefer large seagrass patches, high to medium shoot density, and high seagrass cover. All interviewees said they had noticed a decline in seagrass distribution over the last decade, >20% considered it a large decline. Interviewees also reported decreased numbers of animals, but no change in the number of animal species over the last decade. The main reasons for the decline of seagrass and animals according to interviewees, are an increase in the number of harvesters, and a change in attitude, i.e. people being less careful about the intertidal zone and seagrasses. Invertebrate harvesting was found important for food security and provision of cash income. The current average catch weight was ca. 2 kg/collection day/person, and 3 kg and 5 kg, 5-10 and 30 years ago respectively according to interviewees. At present, the harvesting women earn ca 60-70% and ca 40% of what they would have if catches were the same sizes as they were 5-10 and 30 years ago respectively, according to our calculations. The field sampling within seagrass beds showed that an inaccessible/remote site had significantly higher invertebrate abundance and species richness/diversity than an exploited site (ANOVA). Multivariate statistics further revealed weak but significant differences for animal abundance and biomass between these sites. By combining findings from both interviews and field sampling this study shows that invertebrate harvesters can influence macrofaunal community structure in seagrass meadows, which in turn results in negative impacts on local harvesters' economy and livelihood.

• Newmaster, A.F., Berg, K.J., Ragupathy, S., Palanisamy, M., Sambandan, K., and Newmaster, S. G.  Local knowledge and conservation of seagrasses in the Tamil Nadu State of India.  Journal of Ethnobiology and Ethnomedicine 7: art. 37, 2011.
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  Local knowledge systems of seagrasses are not considered in the conservation of these fragile marine ecosystems. In fact, little is known about the utility of seagrasses in local coastal communities. This is intriguing given that some local communities rely on seagrasses to sustain their lifestyles and have relocated their villages to areas with a rich diversity and abundance of seagrasses. The purpose of this study is to assist in conservation efforts regarding seagrasses through identifying Traditional Knowledge (TK) from local knowledge systems of seagrasses from 40 coastal communities along the eastern coast of India. We explore the assemblage of scientific and traditional local knowledge concerning the 1. classification of seagrasses (comparing scientific and traditional classification systems), 2. utility of seagrasses, 3. traditional Ecological Knowledge (TEK) of seagrasses, and 4. current conservation efforts for seagrass ecosystems. Our results indicate that local knowledge systems consist of a complex classification of seagrass diversity that considers the role of seagrasses in the marine ecosystem. This fine-scaled ethno-classification gives rise to five times the number taxa (10 species = 50 local ethnotaxa), each with its unique role in the ecosystem and utility within these communities including medicine (e.g., treating heart conditions, seasickness etc.), food (nutritious seeds), fertilizer (large amounts of nutrient rich biomass) and livestock feed (goats and sheep). Local communities are concerned about the loss of seagrass diversity and have considerable local knowledge that is valuable for conservation and restoration plans. This study serves as a case study example of the depth and breadth of local knowledge systems for a particular ecosystem that is in peril.

• Collier, C.J., Uthicke, S., and Waycott, M.  Thermal tolerance of two seagrass species at contrasting light levels: Implications for future distribution in the Great Barrier Reef.  Limnology and Oceanography 56(6): 2200-2210, 2011.
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  This study assessed metabolism, growth, and survival of two seagrass species at three different seawater temperatures (27 °C, 30 °C, and 33 °C) under saturating (400 µmol photons m^-2 s^-1) and limiting (40 µmol photons m^-2 s^-1) light over 1 month. Halodule uninervis grown at 33 °C was within its physiological optimum temperature range, exhibiting 2.3× higher photosynthetic rates than at 27 °C, and increased net shoot carbon (C) production (up to 10× higher) at saturating light levels. In contrast, 33°C exceeded the optimum temperature threshold for Zostera muelleri, resulting in critical metabolic imbalances with large reductions in photosynthesis and increases in leaf respiration. This led to substantially lower growth rates (0-2% of those at 27 °C) and lower final biomass (only 10% of that at 27 °C) in the 33 °C treatment after 1month. This decline at higher temperatures occurred at both light levels, but it was more severe in limiting light, where the C balance went into deficit. H. uninervis in the Great Barrier Reef (GBR) exists well within its optimal temperature range and should continue to thrive at projected future temperatures, at least under saturating light levels. In contrast, Z. muelleri currently exists near its upper thermal threshold, and future temperature increases of the magnitude investigated here would likely lead to the contraction of the range of this species from the northern GBR – potentially by more than 1000 km. This could have ecologically significant ramifications, because Z. muelleri is often the only GBR species that currently inhabits muddy estuarine areas, which are critical fisheries habitats.

• Grech, A., Coles, R., and Marsh, H.  A broad-scale assessment of the risk to coastal seagrasses from cumulative threats.  Marine Policy 35(5): 560-567, 2011.
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  Informing the management of coastal marine habitats at broad spatial scales is difficult because of the costs associated with collecting and analyzing ecological data at that scale. Spatially explicit assessments of the risk to coastal marine habitats from cumulative threats provide an alternative approach by identifying sites that are exposed to multiple anthropogenic threats at broad scales. In this study, qualitative measures of vulnerability were combined with geospatial data to evaluate the risk to coastal seagrasses at the scale of the Great Barrier Reef (GBR) region (~26,000 km²) of Queensland, Australia. The risk assessment outputs identified agricultural, urban and industrial runoff, and urban and port developments as the major anthropogenic activities threatening coastal seagrasses. 'Hot spots' with multiple threat exposure were all in industrial port locations and the southern two-thirds of the GBR. There is a distinct discontinuity in threat exposure along the GBR coast with 98% of seagrass meadows in the northern third exposed to only low levels of anthropogenic risk. The clustering of threat exposure is discussed in terms of coastal management policy. The approach outlined in this study provides management agencies a method of achieving maximum return for minimal investment in data collection at broad spatial scales by identifying sites where management intervention would be best targeted.

• Kennedy, H., Beggins, J., Duarte, C.M., Fourqurean, J.W., Holmer, N., Marbà, N., and Middelburg, J.J.  Seagrass sediments as a global carbon sink: Isotopic constraints.  Global Biogeochemical Cycles 24: art. GB4026, 2010.
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  Seagrass meadows are highly productive habitats found along many of the world's coastline, providing important services that support the overall functioning of the coastal zone. The organic carbon that accumulates in seagrass meadows is derived not only from seagrass production but from the trapping of other particles, as the seagrass canopies facilitate sedimentation and reduce resuspension. Here we provide a comprehensive synthesis of the available data to obtain a better understanding of the relative contribution of seagrass and other possible sources of organic matter that accumulate in the sediments of seagrass meadows. The data set includes 219 paired analyses of the carbon isotopic composition of seagrass leaves and sediments from 207 seagrass sites at 88 locations worldwide. Using a three source mixing model and literature values for putative sources, we calculate that the average proportional contribution of seagrass to the surface sediment organic carbon pool is ~50%. When using the best available estimates of carbon burial rates in seagrass meadows, our data indicate that between 41 and 66 gC m^-2 yr^-1 originates from seagrass production. Using our global average for allochthonous carbon trapped in seagrass sediments together with a recent estimate of global average net community production, we estimate that carbon burial in seagrass meadows is between 48 and 112 Tg yr^-1, showing that seagrass meadows are natural hot spots for carbon sequestration.

• Duarte, C.M., Marbà, N., Gacia, E., Fourqurean, J.W., Beggins, J., Barrón, C., and Apostolaki, E.T.  Seagrass community metabolism: Assessing the carbon sink capacity of seagrass meadows.  Global Biogeochemical Cycles 24: art. GB4032, 2010.
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  The metabolic rates of seagrass communities were synthesized on the basis of a data set on seagrass community metabolism containing 403 individual estimates derived from a total of 155 different sites. Gross primary production (GPP) rates (mean ± SE = 224.9 ± 11.1 mmol O₂ m^-2 d^-1) tended to be significantly higher than the corresponding respiration (R) rates (mean ± SE = 187.6 ± 10.1 mmol O₂ m^-2 d^-1), indicating that seagrass meadows tend to be autotrophic ecosystems, reflected in a positive mean net community production (NCP 27.2 ± 5.8 mmol O₂ m^-2 d^-1) and a mean P/R ratio above 1 (1.55 ± 0.13). Tropical seagrass meadows tended to support higher metabolic rates and somewhat lower NCP than temperate ones. The P/R ratio tended to increase with increasing GPP, exceeding, on average, the value of 1 indicative of metabolic balance for communities supporting a GPP greater than 186 mmol O₂ m^-2 d^-1, on average. The global NCP of seagrass meadows ranged (95% confidence limits of mean values) from 20.73 to 50.69 Tg C yr^-1 considering a low global seagrass area of 300,000 km² and 41.47 to 101.39 Tg C yr^-1 when a high estimate of global seagrass area of 600,000 km² was considered. The global loss of 29% of the seagrass area represents, therefore, a major loss of intense natural carbon sinks in the biosphere.

• Berger, V.Ya.  Production of the eel grass Zostera marina Linnaeus, 1753 in the White Sea.  Russian Journal of Marine Biology 37(5): 371-375, 2011.
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  The results of year-round studies of the photosynthesis and respiration rates of the eel grass Zostera marina L. in the White Sea are presented. The annual production of eel grass is estimated to be 9.86 × 10¹¹ kcal. It is concluded that the total annual production of the eel grass constitutes about 3% of the annual phytoplankton production (3 × 10¹³ kcal; according to the data of Bobrov et al., 1995) in the White Sea.

• Jiang, Z.J., Huang, X.-P., and Zhang, J.-P.  Effects of CO₂ enrichment on photosynthesis, growth, and biochemical composition of seagrass Thalassia hemprichii (Ehrenb.) Aschers.  Journal of Integrative Plant Biology 52(10): 904-913, 2010.
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  The effects of CO₂ enrichment on various ecophysiological parameters of tropical seagrass Thalassia hemprichii (Ehrenb.) Aschers were tested. T. hemprichii, collected from a seagrass bed in Xincun Bay, Hainan island of Southern China, was cultured at 4 CO₂(aq) concentrations in flow-through seawater aquaria bubbled with CO₂. CO₂ enrichment considerably enhanced the relative maximum electron transport rate (RETR_max) and minimum saturating irradiance (E_k) of T. hemprichii. Leaf growth rate of CO₂-enriched plants was significantly higher than that in unenriched treatment. Nonstructural carbohydrates (NSC) of T. hemprichii, especially in belowground tissues, increased strongly with elevated CO₂(aq), suggesting a translocation of photosynthate from aboveground to belowground tissues. Carbon content in belowground tissues showed a similar response with NSC, while in aboveground tissues, carbon content was not affected by CO₂ treatments. In contrast, with increasing CO₂(aq), nitrogen content in aboveground tissues markedly decreased, but nitrogen content in belowground was nearly constant. Carbon: nitrogen ratio in both tissues were obviously enhanced by increasing CO₂(aq). Thus, these results indicate that T. hemprichii may respond positively to CO₂-induced acidification of the coastal ocean. Moreover, the CO₂-stimulated improvement of photosynthesis and NSC content may partially offset negative effects of severe environmental disturbance such as underwater light reduction.

• Uhrin, A.V., Kenworthy, W.J., and Fonseca, M.S.  Understanding uncertainty in seagrass injury recovery: an information-theoretic approach.  Ecological Applications 21(4): 1365-1379, 2011.
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  Vessel groundings cause severe, persistent gaps in seagrass beds. Varying degrees of natural recovery have been observed for grounding injuries, limiting recovery prediction capabilities, and therefore, management's ability to focus restoration efforts where natural recovery is unlikely. To improve our capacity for predicting seagrass injury recovery, we used an information-theoretic approach to evaluate the relative contribution of specific injury attributes to the natural recovery of 30 seagrass groundings in Florida Keys National Marine Sanctuary, Florida, USA. Injury recovery was defined by three response variables examined independently: (1) initiation of seagrass colonization, (2) areal contraction, and (3) sediment in-filling. We used a global model and all possible subsets for four predictor variables: (1) injury age, (2) original injury volume, (3) original injury perimeter-to-area ratio, and (4) wave energy. Successional processes were underway for many injuries with fast-growing, opportunistic seagrass species contributing most to colonization. The majority of groundings that exhibited natural seagrass colonization also exhibited areal contraction and sediment in-filling. Injuries demonstrating colonization, contraction, and in-filling were on average older and smaller, and they had larger initial perimeter-to-area ratios. Wave energy was highest for colonizing injuries. The information-theoretic approach was unable to select a single "best" model for any response variable. For colonization and contraction, injury age had the highest relative importance as a predictor variable; wave energy appeared to be associated with second-order effects, such as sediment in-filling, which in turn, facilitated seagrass colonization. For sediment in-filling, volume and perimeter-to-area ratio had similar relative importance as predictor variables with age playing a lesser role than seen for colonization and contraction. Our findings confirm that these injuries naturally initiate seagrass colonization with the potential to recover to pre-injury conditions, but likely on a decadal scale given the slow growth of the climax species (Thalassia testudinum), which is often the most severely injured. Our analysis supports current perceptions that sediment in-filling is critical to the recovery process and indicates that in order to stabilize injuries and facilitate seagrass recovery, managers should consider immediate restorative filling procedures for injuries having an original volume >14-16 m³.

• Christianen, M.J.A., van der Heide, T., Bouma, T.J., Roelofs, J.G.M., van Katwijk, M.M., and Lamers, L.P.M.  Limited toxicity of NH_x pulses on an early and late successional tropical seagrass species: Interactions with pH and light level.  Aquatic Toxicology 104(1-2): 73-79, 2011.
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  Seagrasses have declined at a global scale due to light reduction and toxicity events, caused by eutrophication and increased sediment loading. Although several studies have tested effects of light reduction and toxicants on seagrasses, there is at present no information available on their interacting effects. In a full-factorial 5-day laboratory experiment, we studied short-term interactive effects of light conditions, pH and reduced nitrogen (NH_x) in the water layer, mimicking pulses of river discharge, on the tropical early successional species Halodule uninervis and the late successional species Thalassia hemprichii. In contrast to recent results reported for the temperate species Zostera marina, increased NH_x supply did not affect leaf mortality or photochemical efficiency in H. uninervis and in 7 out of 8 treatments for T. hemprichii. However, both tropical species demonstrated striking differences in nitrogen accumulation, free amino acid composition and free NH₃ accumulation. The increase in tissue nitrogen content was two times higher for H. uninervis than for T. hemprichii. Nitrogen stored as free amino acids (especially asparagine) only increased in H. uninervis. High pH only affected T. hemprichii, but only when not shaded, by doubling its free NH₃ concentrations, concomitantly decreasing its photosynthetic efficiency. Our results indicate that the early successional H. uninervis has higher tolerance to high NH_x loads as compared to the late successional T. hemprichii. H. uninervis was better able to avoid toxic internal NH_x levels by further assimilating glutamine into asparagine in contrast to T. hemprichii. Moreover, both tropical species seem to cope much better with high NH_x than the temperate Z. marina. The implications for the distribution and succession of seagrass species under high nutrient loads are discussed.

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