Understanding trophodynamics of estuarine and marine ecosystems is important in developing food-web models prevalent in ecosystem-based approaches to fisheries management, as well as assessing ecosystem condition and function. Few studies have examined seasonal and spatial trophodynamics of nekton assemblages in shallow subtropical waters on estuary-wide spatial extent. We used fisheries-independent monitoring data to examine the spatial distribution and seasonal trophodynamics of nekton assemblages in shallow waters (≤ 1.5m) of Charlotte Harbor, Florida, USA, as well as the lower Peace and Myakka Rivers. Overall nekton densities were generally high in spring (April - May), especially in the lower rivers, and declined through the summer (June – October), and fall (November – December), with small increases in mean nekton densities in winter (January – March). We found that species composition as well as trophic structure of nekton assemblages changed along a broad spatial gradient from the lower portions of the Peace and Myakka Rivers to lower Charlotte Harbor near the passes to the Gulf of Mexico. Nekton assemblages dominated by planktonic-feeding species were located in lower Peace and Myakka Rivers and extended into upper Charlotte Harbor in apparent response to seasonal fluctuations in freshwater inflow. In contrast, most of the nekton assemblages within Charlotte Harbor proper were dominated by nekton species feeding within the benthos (those feeding on benthic infauna, epifauna, seagrass, and/or detritus) throughout the year. The proportion of benthic feeders was positively correlated with landscape metrics describing the area of continuous seagrass beds mapped from aerial photography, and negatively correlated with distance to the nearest pass to the Gulf of Mexico. These relationships were consistent throughout all four seasons of the year. The proportion of benthic feeders varied seasonally, and was relatively high in the fall and winter compared with the spring and summer. The predominance of benthic feeding species during fall and winter when overall nekton densities were low, suggests that benthic production is an important in supporting populations of forage fishes for higher trophic levels, such as piscivorous fishes and wading birds during this time of year. This is an important time for overwintering wading birds, as well as several economically-important fishery species who are recruiting into shallow water areas of Charlotte Harbor. Planktonic production is often the focus of food-web models with benthic production sometimes not included. We found both planktonic and benthic production to be important in Charlotte Harbor. Therefore, both sources of production need to be included in food-web models for Charlotte Harbor, as well as similar subtropical estuaries with relatively large areas of seagrass beds.
Trophodynamics of nekton assemblages and relationships with estuarine habitat structure across a subtropical estuary