Our evolving conceptual model of the coastal eutrophication problem

A primary focus of coastal science during the past 3 decades has been the q uestion: How does anthropogenic nutrient enrichment cause change in the str ucture or function of nearshore coastal ecosystems? This theme of environme ntal science is recent, so our conceptual model of the coastal eutrophicati on problem continues to change rapidly. In this review, I suggest that the early (Phase I) conceptual model was strongly influenced by Limnologists, w ho began intense study of lake eutrophication by the 1960s. The Phase I mod el emphasized changing nutrient input as a signal. and responses to that si gnal as increased phytoplankton biomass and primary production, decompositi on of phytoplankton-derived organic matter, and enhanced depletion of oxyge n from bottom waters. Coastal research in recent decades has identified key differences in the responses of lakes and coastal-estuarine ecosystems to nutrient enrichment, The contemporary (Phase II) conceptual model reflects those differences and includes explicit recognition of (1) system-specific attributes that act as a filter to modulate the responses to enrichment (le ading to large differences among estuarine-coastal systems in their sensiti vity to nutrient enrichment); and (2) a complex suite of direct and indirec t responses including linked changes in: water transparency, distribution o f vascular plants and biomass of macroalgae, sediment biogeochemistry and n utrient cycling, nutrient ratios and their regulation of phytoplankton comm unity composition, frequency of toxic/harmful algal blooms, habitat quality for metazoans, reproduction/growth/survival of pelagic and benthic inverte brates, and subtle changes such as shifts in the seasonality of ecosystem f unctions. Each aspect of the Phase II model is illustrated here with exampl es from coastal ecosystems around the world. In the last section of this re view I present one vision of the next (Phase III) stage in the evolution of our conceptual model, organized around 5 questions that will guide coastal science in the early 21st century: (1) How do system-specific attributes c onstrain or amplify the responses of coastal ecosystems to nutrient enrichm ent? (2) How does nutrient enrichment interact with other stressors (toxic contaminants, fishing harvest, aquaculture, nonindigenous species, habitat loss, climate change, hydrologic manipulations) to change coastal ecosystem s? (3) How are responses to multiple stressors linked? (4) How does human-i nduced change in the coastal zone impact the Earth system as habitat for hu manity and other species? (5) How can a deeper scientific understanding of the coastal eutrophication problem be applied to develop tools for building strategies at ecosystem restoration or rehabilitation?