Eutrophication in shallow coastal bays and lagoons: the role of plants in the coastal filter

Nutrient loading to coastal bay ecosystems is of a similar magnitude as that to deeper, river-fed estuaries, yet our understanding of the eutrophication process in these shallow systems lags far behind. In this synthesis, we focus on one type of biotic feedback that influences eutrophication patterns in coastal bays—the important role of primary producers in the ‘coastal filter’. We discuss the 2 aspects of plant-mediated nutrient cycling as eutrophication induces a shift in primary producer dominance: (1) the fate of nutrients bound in plant biomass, and (2) the effects of primary producers on biogeochemical processes that influence nutrient retention. We suggest the following generalizations as eutrophication proceeds in coastal bays: (1) Long-term retention of recalcitrant dissolved and particulate organic matter will decline as seagrasses are replaced by algae with less refractory material. (2) Benthic grazers buffer the early effects of nutrient enrichment, but consumption rates will decline as physico-chemical conditions stress consumer populations. (3) Mass transport of plantbound nutrients will increase because attached perennial macrophytes will be replaced by unattached ephemeral algae that move with the water. (4) Denitrification will be an unimportant sink for N because primary producers typically outcompete bacteria for available N, and partitioning of nitrate reduction will shift to dissimilatory nitrate reduction to ammonium in later stages of eutrophication. In tropical/subtropical systems dominated by carbonate sediments, eutrophication will likely result in a positive feedback where increased sulfate reduction and sulfide accumulation in sediments will decrease P adsorption to Fe and enhance the release of P to the overlying water.