The Chowan River in North Carolina is subject to two environmental management efforts: nutrient reduction intended to curtail the occurrence and severity of phytoplankton blooms and a moratorium on river herring harvest intended to allow these highly depleted species to recover. It is unknown to what extent zooplanktivorous herring may indirectly affect phytoplankton through zooplankton consumption, but this positive effect on phytoplankton abundance could off-set nutrient management efforts. This study investigated the possibility of conflicting results of these management actions in the Chowan River. We examined indirect evidence for the strength of top-down and bottom-up forcing through analysis of historic records of nutrients, phytoplankton, zooplankton, and river herring. Change in river herring diet between 1982 and 2008 was examined to provide direct evidence of the effect of herring on zooplankton. Zooplankton community composition in two mainstem and two tributaries of the Chowan River was examined over a one year period as part of an ongoing study on the abundance of forage for river herring.
Results/Conclusions
Decreases in concentrations of ammonium (46%), nitrate (52%), and phosphorus (18%) occurred between 1982-1983 and 2005-2006, and herring spawning stock biomass fell 98% between these two periods. Chorophyll a, a proxy for phytoplankton biomass, decreased 60%, while crustacean zooplankton increased 6-fold. All changes were statistically significant (α=0.05). Preliminary data suggest that consumption of zooplankton by larval herring was similar between the 1982-1983 period and 2008-2009, with the number of prey per fish averaging 195 and 146, respectively. We expect that the increased zooplankton abundance was a result of decreased consumption by herring, and the decrease in phytoplankton may be due to a combination of both decreased resources and increased grazing losses. We predict that locations within the Chowan River basin with a larger percentage of crustacean zooplankton, such as the upper Chowan River and the Wiccacon River, are more likely to show increased phytoplankton abundance as a result of river herring restoration. These observations and hypotheses will guide further research on the potential for river herring restoration to indirectly affect phytoplankton in nutrient sensitive coastal rivers.
