Eutrophication-Driven Shifts in Primary Producers in Shallow Coastal Systems: Implications for System Functional Change

Significant progress has been made recently towards a better understanding of the nature, causes, and consequences of anthropogenic eutrophication of shallow coastal systems. It is well established that, in pristine systems dominated by seagrasses, incipient to moderate eutrophication often leads to the replacement of seagrasses by phytoplankton and loose macroalgal mats as the dominant producers. However, less is known about the interactions between phytoplankton and loose macroalgae at intense eutrophication. Using a combination of original research and literature data, we provide support for the hypothesis that substantial macroalgal decline may occur at intense eutrophication due to severe water column shading. Our results suggest that such declines may be widespread. However, we also show that intense eutrophication is not always necessarily conducive to severe water column shading and large macroalgal declines, possibly due to short water residence time and/or elevated grazing on phytoplankton. Furthermore, we provide support to the hypothesis that the occurrence of hypoxic/anoxic conditions in eutrophication-driven shifts in dominant primary producer assemblages influences the nature and extent of functional change in the system. Focusing on the macroalgal blooms and seagrass decline that often occur at incipient/moderate eutrophication, we show the blooms have a positive effect on epifaunal abundance under well-oxygenated conditions, but a negative effect if pervasive anoxic/hypoxic conditions develop with the bloom. These findings provide support to prior suggestions that secondary productivity in shallow coastal systems may increase as seagrasses get replaced by loose macroalgal stands if the stands remain well oxygenated. In concert, our results contribute to an improvement of our current model of eutrophication of shallow coastal systems and suggest that further effort should be put on ascertaining the mechanisms that may prevent severe water column shading and large macroalgal decline at intense eutrophication, as well as thorough documentation of the impacts of anoxic/hypoxic conditions on system functionality at different stages of eutrophication.