Estuarine and coastal transformation is as old as civilization yet has dramatically accelerated over the past 150 to 300 years. Reconstructed time lines, causes, and consequences of change in 12 once diverse and productive estuaries and coastal seas worldwide show similar patterns: Human impacts have depleted >90% of formerly important species, destroyed >65% of seagrass and wetland habitat, degraded water quality, and accelerated species invasions. Twentieth-century conservation efforts achieved partial recovery of upper trophic levels but have so far failed to restore former ecosystem structure and function. Our results provide detailed historical baselines and quantitative targets for ecosystem-based management and marine conservation.

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Depletion, Degradation, and Recovery Potential of Estuaries and Coastal Seas

We present an analysis of extensive nutrient data sets from two river-dominated coastal ecosystems, the northern Adriatic Sea and the northern Gulf of Mexico, demonstrating significant changes in surface nutrient ratios over a period of 30 years. The silicon:nitrogen ratios have decreased, indicating increased potential for silicon limitation. The nitrogen:phosphorus and the silicon:phosphorus ratios have also changed substantially, and the coastal nutrient structures have become more balanced and potentially less limiting for phytoplankton growth. It is likely that net phytoplankton productivity increased under these conditions and was accompanied by increasing bottom water hypoxia and major changes in community species composition. These findings support the hypothesis that increasing coastal eutrophication to date may be associated with stoichiometric nutrient balance, due to increasing potential for silicon limitation and decreasing potential for nitrogen and phosphorus limitation. On a worldwide basis, coastal ecosystems adjacent to rivers influenced by anthropogenic nutrient loads may experience similar alterations.

Changes in nutrient structure of river-dominated coastal waters: stoichiometric nutrient balance and its consequences

Stoichiometric nutrient balance and origin of coastal eutrophication

The eutrophication, or nutrient enrichment, of coastal waters as a result of man’s activities is now widely recognized as a major, world-wide pollution threat. Essentially, the increased anthropogenic source of inorganic plant nutrients interferes with the natural annual nutrient cycles and can artifically enhance primary production during periods when activity is normally low. This can have quite considerable ecological consequences for both pelagic and benthic organisms. For example, phytoplankton activity will be increased (Hoogweg et al. 1991) and, although this can be generally beneficial by increasing fisheries (Raymont 1947; Fonselius 1978; Elmgren 1989), there is some evidence that it has resulted in the occurrence of some phytoplankton blooms, both toxic and non-toxic, which have had serious effects on local fisheries and leisure activities (Braarud 1945; Ruud 1968; O’Sullivan 1971; Zou and Dong 1983; Rosenberg 1985; Kimor 1991). One such bloom occurred in the northern Adriatic in 1988 when large quantities of mucilaginous material was washed up on many tourist beaches (Degobbis 1989; Vukadin 1991). A number of authors have similarly speculated on the possible relationship between eutrophication and the occurrence of toxic blooms of microalgae in the North Sea (Cole 1972).

The Occurrence of “Green Tides”— a Review

The Mediterranean Sea has been strongly influenced by human activities for millennia. Although the environmental history of its surrounding terrestrial ecosystems has received considerable study, historical changes in its marine realm are less known. We used a multidisciplinary approach combining paleontological, archeological, historical, fisheries, and ecological data to reconstruct past changes in marine populations, habitats, and water quality in the Adriatic Sea. Then, we constructed binary food webs for different historical periods to analyze possible changes in food-web structure and functioning over time. Our results indicate that human activities have influenced marine resource abundance since at least Roman times and accelerated in the nineteenth and twentieth centuries. Today, 98% of traditional marine resources are depleted to less than 50% of former abundance, with large (>1 m) predators and consumers being most affected. With 37% of investigated species rare and 11% extirpated, diversity has shifted towards smaller, lower trophic-level species, further aggravated by more than 40 species invasions. Species providing habitat and filter functions have been reduced by 75%, contributing to the degradation of water quality and increased eutrophication. Increased exploitation and functional extinctions have altered and simplified food-web structure over time, especially by changing the proportions of top predators, intermediate consumers, and basal species. Moreover, simulations of species losses indicate that today’s ecosystems may be less robust to species extinctions than in the past. Our results illustrate the long-term and far-reaching consequences human activities can have on marine food webs and ecosystems.

G. Crosa

Papers

Nutrient load carried by the River Po into the Adriatic Sea, 1968–1987