Response of phytoplankton communities to nitrogen input reduction in mesocosm experiments

The effects of a reduction in inorganic nitrogen input on phytoplankton communities (species composition, biomass and production) were studied in mesocosms. Experiments were conducted in 6 land-based mesocosms consisting of 3 m(3) black polyethylene tanks. Light climate, temperature, mixing regime, and P and Si input were regulated to reproduce conditions comparable to those in Dutch coastal waters. Three levels of nitrogen availability were applied to the mesocosms used as replicates in 2 experiments, of 4 wk, in May-June and June-July 1994. In Expt 1, after 10 d in all treatments, diatoms (mainly Nitzschia delicatissima) became Si-Limited and flagellates dominated phytoplankton. Within the group of flagellates, Phaeocystis sp, dominated numerically in the last week of the experiment when N was Limiting and irradiance had increased. Phaeocystis sp, concentrations were higher in the mesocosms with the highest nitrogen load. In Expt 2, after the collapse of the initial diatom bloom due to Si-Limitation (Thalassiosira sp.), an N-Limited Phaeocystis sp. bloom developed and dominated the flagellate community. The decline of the Phaeocystis sp, bloom occurred together with the development of a second diatom bloom (Leptocylindrus danicus) under N-limitation. Phytoplankton biomass and production in these experiments reached levels measured in MERL mesocosm experiment for similar N-additions. The response of primary production to N-additions was tuned by the availability of Light and other macronutrients. A saturation model could be fitted to observations with a set of parameters corresponding to representative ecophysiological characteristics. Phytoplankton species composition was related to season and resembled field communities observed in similar periods of the year. N-Limited Phaeocystis colonial blooms were observed in mesocosms where 50% of the N-pool consisted of NH4+.