Beyond bulk properties: Responses of coastal summer plankton communities to nutrient enrichment in the northern Baltic Sea

Enclosure experiments with all combinations of NH 4 and PO 4 treatments were carried out in the northern Baltic coastal zone during the post‐spring bloom growth season. Empirical models were used to identify the important 3‐d treatment effects on the state and process variables governing the main plankton compartments. Moreover, dilution experiments revealed the main nutrient sources for algal growth (external pools, intracellular stores, remineralization) and estimated phytoplankton growth and feeding loss rates. Maximal (nutrient‐replete) algal community growth rates were 0.1‐0.7 d ‐1 . Small (2‐10 µm) algae grew fast (0.5‐1.5 d ‐1 ), whereas the abundant trichal blue‐green algae grew very slowly. Ciliates showed a clear increase in units with positive algal response at minimum grazing estimates of 0.05‐0.3 d ‐1 . Initial inorganic N: P ratios were consistently low (<4, wt/wt), and internal stores and remineralization were the most important nutrient sources for algae. During P‐replete early summer, phytoplankton showed N limitation of biomass, rather than growth rate, in all (<2, 2‐10, >10 µm) size fractions. During the mineral N‐ and P‐deplete late summer bloom of trichal blue‐green algae, the positive P responses of chlorophyll a‐normalized 14 CO 2 fixation and growth rates indicated physiological P deficiency of the whole algal assemblage, but only a combined N and P addition evoked an increase in algal biomass. In early summer, low inorganic N: P ratios reflected N limitation, as commonly found in estuaries, but later, the indicative value of this ratio evaporated, evidently because of gaseous N 2 fixation by blue‐green algae.