Change in the World's Oceans

by Bruce McKay

Originally published in International Oceans Newsletter, no. 1, 1993.

---

An explosion in number of crown-of-thorns starfish is decimating sections of Australia's Great Barrier Reef. The mysterious and fatal "withering syndrome" of black abalone continues in Southern California. A "wasting disease" is striking eelgrass along northeastern U.S. coastlines.

Are these natural cyclical occurrences or symptoms of regional human induced disturbances? How does one explain the epidemic of the cancerlike fibropapilloma disease, a disease virtually unheard of ten years ago, in green turtle populations in Florida and Hawaii? What is the reason for the virtual disappearance of black sea urchin throughout much of the Caribbean? Can we expect an increase in toxic algae events of the sort that killed 14 endangered humpback whales along the U.S. mideastern shore during late 1987/early 1988 or some 37 endangered manatees in Florida water during 1982? Are the increases in "ulcerative syndromes" in coastal finfish in Southeast Asia, the U.S. and Europe just an artifact of increased surveillance or an ominous sign of extensive coastal degradation? To what extent is commercial fishing responsible for the massive declines in populations of Stellar sea lions, harbour seals and fur seals in Alaskan waters?

These are just some of the questions with which marine scientists are faced as both the popular press and scientific literature continue to be filled with reports of unusual events and descriptions of significant change occurring in marine environments.

One of the most striking phenomena has been what appears to be a remarkable increase in the frequency and geographic extent of toxic phytoplankton blooms or "red tides". Indeed, human health concerns and the enormity of the ecological and economical impacts of blooms is the drawing card for some 350 scientists to the upcoming sixth International Conference on Toxic Marine Phytoplankton. Only a couple of years ago, such an event would have drawn but a few dozen participants.

Although planktonic blooms are a natural occurrence in aquatic environments, many individual phytoplankton species are stimulated by nutrient loadings. The over-enrichment or eutrophication of coastal waters by agricultural run-off, human and farm animal waste, or by the nitrogenous element of "acid" rain are likely important components in at least some bloom occurrences. The alteration of food web dynamics by chemical contaminants and over-fishing may also be a factor. The transfer of aquaculture products (e.g. contaminated bivalves) or ballast water discharge adequately explains the surprise appearance of species in regions where they were historically absent.

The accidental, and at time purposeful, introduction of "alien" species into marine environments has, in this regard, become a global issue. Besides eroding the biogeographical distinctness of coastal environments, introduced species can cause considerable ecological damage. For example, Caulerpa taxifolia , one of at least nine of the seaweed species introduced to the Mediterranean, may gradually replace ecologically important Posidonia oceanica seagrass beds, which are already declining under pressure from pollution and land reclamation in the region. Surprisingly, the species can grow larger and produce more toxin in the Mediterranean than it ever could in its native tropical environments. This has led to suggestions that it may be evolving new genotypic traits helping it to out compete existing species.

But whether it be C. taxifolia flourishing in the Mediterranean, or the European shore crab scuttling along the South African coast, or the Asian clam that now makes up most of the living biomass in San Francisco Bay, this much is known: nearshore environments around the world are irrevocably altered by a virtual onslaught of alien invaders.

Changes in coastal environments are occurring, of course, in numerous other ways; this is largely a result of the fact that over half of the world's population lives along the coastal zone. Highly productive wetlands such as seagrass beds, saltwater marshes, coral reefs and mangrove forests are under tremendous assault. For example, over half--and perhaps as much as seventy percent--of mangrove forests in Southeast Asia, South Asia and the Caribbean have been destroyed over the last 40 years. Changes in these ecosystems are now so extensive that the regeneration of mangroves into former habitats is unlikely. Bereft of this natural physical barrier, those coastal environments will be particularly vulnerable to sea level rise and extreme storm events as predicted by some climate change scenarios.

Increasingly, many coral reef systems are falling victim to reef fisheries, eutrophication, and sedimentation. Some estimates have suggested that up to eighty percent of coral reefs in SE Asia have been severely degraded as a result of human activity. In addition, an unprecedented series of "coral bleaching" events (i.e., in which there is a dissociation of the symbiotic relationship between the color-providing algae or zooxanthellae and the host coral) have struck in a variety of regions in the last 15 years. Bleached coral, though not always dead, appear to be more vulnerable to other stresses, which in turn can lead to reduced growth and mortality. The wide geographic range of coral bleaching complexes and coral's known dependence on a narrow temperature regime has resulted in speculation that this may be one early sign of increased climatic warming.

Some startling events in populations of higher-level predators have also been occurring. Over the past six years there have been four major die-off events involving marine mammals. During 1987/88 over half of the inshore population of US east coast bottlenose dolphin succumbed, ultimately, to massive bacterial infections. Shortly after, in 1988, approximately 18,000 harbour seals died in northern European waters as a result of infection by a distemper-like virus. During 1990 and 1991, another distemper-like virus raced through striped dolphin populations in the Mediterranean; thousands of animals were estimated to have died. And during the first four months of 1990, 270 bottlenose dolphins washed ashore along the US side of the Gulf of Mexico. In all cases high levels of persistent and highly toxic organochlorine chemicals were found in the affected populations, leading to much speculation that contaminants may have played an important role in the events.

There are numerous other human-related impacts than those mentioned above that are currently resulting, or will result in the future, in extensive alterations and stress to marine environments. These include: climate change; increased UV radiation; the damming of rivers for irrigation and energy purposes; overfishing and other harmful fishing practices; sea surface microlayer contamination by a wide variety of contaminants; and the myriad development and maintenance projects along coastlines worldwide.

In many cases, the ecological significance of these and other events, changes, and conditions currently defy understanding. Perhaps, they may simply portend the continued alteration and reorganisation of coastal and oceanic environments, much of it at the hand of humankind. After all, the regenerative and adaptability potential of many environments is enormous.

But at its worst, we may be witnessing the unraveling of delicate trophic balances and pathways, and of ecosystemic regulatory controls, with a syndrome of structural effects to be felt over larger and larger regions.