Effects of grazer and substrate amendments on nutrient and plankton dynamics in estuarine enclosures

We compared the effects of substrate C:N ratio and macrozooplankton activity on nutrient and chlorophyll dynamics by amending the substrate C:N ratios in carboys containing natural estuarine microplankton (<200 mu m) with additions of glucose (High C:N), arginine (Low C:N), or nothing (Control). Adult copepods (Acartia tonsa, 10 ind. l(-1)) were added to 1 carboy of each substrate treatment. Water and copepods were collected from the Choptank River, a subestuary of the Chesapeake Bay (USA) in August 1993. Ambient concentrations of NH4+, NO3-, and o-PO43- (orthophosphate) and dissolved primary amines (DPA) were all <2.0 mu g-at. l(-1). Dissolved and particulate nutrients and pigments were monitored over a 2 d period. In all carboys, plankton shifted from being net consumers of nutrients during the first 17 to 23 h (Phase 1) to being net regenerators afterwards (Phase 2). Chlorophyll concentrations declined and phaeopigments increased throughout the experiment. Both substrates stimulated microbial activity, as indicated by decreased accumulation of o-PO43- during Phase 2, increased accumulation of particulate carbon (PC) and nitrogen (PN) and increased chlorophyll loss during nighttime in the High C:N and Low C:N carboys relative to the Control carboy. In addition, the low C:N substrate resulted in increased accumulations of NH4+, NO2- and NO3-; increased chlorophyll concentration; and a day/night pattern in chlorophyll concentration. Copepod additions resulted in greater PC, PN and chlorophyll losses and day/night patterns in chlorophyll concentration. The additions of copepods and substrates together resulted in several interactive effects, most notably, increased accumulations of NH4+, o-PO43-, and, in the High C:N treatment, NO2-+NO3-, and greater chlorophyll, PC, and PN losses. Estimated rates of ingestion and excretion by the added copepods could not account for the observed changes in chlorophyll and nutrients, especially in the carboys with copepod and substrate additions, suggesting that the copepods increased nutrient regeneration and phytoplankton removal by microzooplankton.