ABSTRACT
Estuaries associated with alluvial rivers are highly productive because of nutrient loading and associated high phytoplankton productivity. In the northeastern Gulf of Mexico, phosphorus is limiting in such areas during winter-spring periods whereas phosphorus and nitrogen are often co-limiting during warmer months of the year. However, secondary productivity, in the form of infauna (animals living in the sediments), epibenthic macroinvertebrates, and fishes is not evenly distributed. The highest levels of such productivity are located in the immediate receiving areas at the heads of alluvial estuaries (Figures 8.1 and 8.2). These areas represent primary nursery grounds for shrimp, crabs, and sciaenid fishes. Freshwater influxes thus determine the distribution of primary and secondary production in alluvial estuaries in addition to controlling important habitat variables. However, trophic linkages take precedence over habitat distribution, and estuarine organisms are concentrated in areas receiving freshwater influxes as a trophic response to enhanced phytoplankton productivity. When anthropogenous nutrient loading causes an imbalance in the phytoplankton communities, there are resultant alterations in the associated food webs, many of which are benthic. Such changes can eventually result in lost secondary production in the most productive parts of the system. Altered phytoplankton associations are thus translated into losses of benthic productivity. Preliminary analyses link plankton blooms with losses of benthic microalgae. Although the exact mechanisms of such effects on the sediments remain largely unknown, the association of the quality of planktonic communities with effects on benthic productivity could be an important factor in the observed reduction of secondary production in coastal systems affected by cultural eutrophication. These effects can be subtle and, without detailed information concerning phytoplankton community response to seasonal and interannual trends of nutrient loading, significant changes in the overall productivity of coastal systems can occur without detection.
