Edinburgh Napier University

  During 1999 ex-situ microcosm experiments were carried out at Dunstaffnage Marine Laboratory, Oban using natural assemblages of microplankton (< 200 m in size) in a series of enrichment experiments using continuous culture techniques to determine q - the yield of chlorophyll from dissolved available inorganic nitrogen (DAH\I), during and after an enrichment event. The experiments lasted between 11 and 14 days and produced a time series of q after an enrichment of either 12 M ammonium or 12 M nitrate. All
other essential nutrients, vitamins and trace metals were added in excess so that only DAIN would limit chlorophyll synthesis. Experiments were carried out during Spring, Summer and Autumn and environmental regimes in the growth room were set-up to
mimic ambient conditions at the sampling site for each season. Water samples were collected every 2 days and were analysed for chlorophylls a, b and c, chlorophyll a breakdown products, carotenoids, dissolved inorganic nutrients and particulate nitrogen
and carbon. Identification and enumeration of microplankton was also undertaken.
During 2 of the experiments nitrogen isotope techniques were used to determine uptake rates for nitrate and ammonium. The results indicated that the microplanktonic response to an enrichment event could be divided into 3 phases: Phase I q - a rapid uptake of DAIN
and the synthesis of large quantities of pigments; Phase H q - DAIN became limiting and there was a decline in q caused by nutrient limitation and an increase in grazing pressure; Phase II q - after declining q remained fairly stable. Nitrogen tied-up in autotrophic
biomass was transferred to the heterotrophs as grazing pressure and algal death increased accompanied by a calculated rise in dissolved organic nitrogen through degradation processes (assuming mass conservation of nitrogen was occurring in the microcosms),
and, presumably, regenerated DAIN. There were seasonal differences in q caused by changing environmental conditions such as light, temperature and background nutrient concentrations. Seasonal changes in the community structure of microplankton collected from the sampling site could also have affected the value of q. During the Summer and Autumn Experiments ammonium enriched microcosms produced lower values of q compared to nitrate enriched microcosms but further investigation is needed to clarify the reasons for this. The method used to estimate chlorophyll or nitrogen had an effect on the value of q. Refined values of q have been produced for use in the screening model used to predict potential eutrophication in the UK.