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 ac tivity 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 no thing (Control). Adult copepods (Acartia tonsa, 10 ind. l(-1)) were ad ded to 1 carboy of each substrate treatment. Water and copepods were c ollected 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 mon itored 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 be ing net regenerators afterwards (Phase 2). Chlorophyll concentrations declined and phaeopigments increased throughout the experiment. Both s ubstrates stimulated microbial activity, as indicated by decreased acc umulation of o-PO43- during Phase 2, increased accumulation of particu late carbon (PC) and nitrogen (PN) and increased chlorophyll loss duri ng nighttime in the High C:N and Low C:N carboys relative to the Contr ol carboy. In addition, the low C:N substrate resulted in increased ac cumulations of NH4+, NO2- and NO3-; increased chlorophyll concentratio n; and a day/night pattern in chlorophyll concentration. Copepod addit ions resulted in greater PC, PN and chlorophyll losses and day/night p atterns in chlorophyll concentration. The additions of copepods and su bstrates together resulted in several interactive effects, most notabl y, increased accumulations oi NH4+, o-PO43-, and, in the High C:N trea tment, NO2-+NO3-, and greater chlorophyll, PC, and PN losses. Estimate d rates of ingestion and excretion by the added copepods could not acc ount 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.