The aim of t h~s study was to quantify zooplankton feeding interactions and copepod reproduction dunng blooms of dlatoms and flagellates (lncludlng E m~l l a n~a huxleyl) In fertilised mesocosms A number of mlcrozooplankton grazlng (dilution senes), copepod f e e d~n g (bottle ~ncubation) and egg productlon experiments were performed during a 4 \vk summer penod Mlcrozooplankton (malnly ciliates) peaked dunng an initial bloom d o m~n a t e d by the diatom Skeletonema costatum and flagellates 210 pm w h~c h apparently became grazer-controlled Mauimum grazing rates were 1 5 to 1 8 d.' for diatoms, the calcifying haptophyte Emlllan~a huxleyl and flagellates 2 to 10 pm, and 65 to 80% of the average standing stock of these algae were removed dally Dunng a subsequent bloom of E h u x l e y~ the m~crozooplankton composition changed and ~t s biomass decreased to <25 % and the daily turnover of dlatoms and E huxleyl fell to ca 50% In contrast to other algae E huxleyl spec~fic growth was never surpassed by microzooplankton grazlng The copepod C finn~archlcus (CV and CV1 females) preferred cihates 230 pm, but ciliates <30 pm, dlatoms and rotlfers were also occas~onally preyed upon at h~g h rates E h u x l e y~ was barely Ingested at low concentrations (0 4 to 6 X 105 cells I-') but was cleared at 106 m1 lnd -' d ' at peak concentrations (1 2 X 107 cells I-') It then made up 74 % of total carbon Ingestion Although copepod lngest~on rates were slmilar dunng blooms of diatoms and E huxleyl, egg productlon rates were s~gnlficantly higher dunng blooms of the latter, and mesozooplankton biomass increased 3 tlmes more In mesocosms dominated by E huxleyl compared to mesocosms w~t h dlatom blooms at slmllar algal blomass Impact by copepods on the phytoplankton development was m a n l y induced lndlrectly by selective predation on the m~crozooplankton A method to correct copepod feeding rate measurements for errors d u e to loss of n~~crozooplankton grazing In the ~n c u b a t~o n bottles IS presented

Sw~tching between algal (Thalasdosira we~ssflog~i) and ciliate (Strobilidium undinum) food by the marine copepod Acartia clausi was investigated In the laboratory by short incubation experiments with l4C-labeled prey. A. clausi displayed a Holling type 3 functional response (which differed significantly from a type 2 response, p c 0.05) for ciliates when there was a constant abundance of algae present, and likewise for algae when there was a constant abundance of ciliates present. The results were implemented in a mathematical model to investigate the effect of different functional responses on a simple food web comprised of nutrient, algae, ciliates and copepods. In the model, ciliates and copepods competed for resources (algae) and ciliates were also prey for copepods. This blend of predation and competition among copepods and ciliates corresponds to intraguild predation as defined by Polis & Holt (1992; Trends Ecol Evol 7:151-154). Stable solutions with all state variables present were found over a range of nutrient concentrations when the copepods displayed type 3 functional responses. On the contrary, when copepods displayed type 2 responses, such stable solutions were only found at very low input nutrient concentrations. Coexistence of ciliates and copepods further required that clliates had a lower threshold prey concentration for positive net growth than copepods.

A total of 6 planktonic naked ciliates were isolated from the Oslofjord, Norway. Numerical and functional responses were investigated using bottle incubations at 11 to 14°C, with the prasinophyceae Nephroselmis pyriformis and cryptophyceae Hemiselmis sp. as food. Growth rates followed hyperbolic curves, which levelled off at about 15 000 algal cells ml -1 (152 µg C l -1). Highest growth rate was recorded for the large heterotroph Strobilidium spiralis (μ max = 1.38 ± 0. ). Food thresholds for positive net growth rates were estimated to be in the range of 600 to 3300 algal cells ml . Specific ingestion rates increased at increasing food concentrations, and maximum measured rates were generally in the range of 2 to 4 d -1 . S. spiralis increased its clearance rate to a maximum of 18.3 ± 0.69 µl h -1 as algal concentrations were enhanced from 900 to 2600 algal cells ml -1. Gross growth efficiency ranged from 7 to 52% above threshold food levels, with a mean and median of 22 and 23%, respectively.

Enclosure experiments with all combinations of NH4 and PO4 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 14CO2 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 N2 fixation by blue‐green algae.

AcknowledgmentsThanks to our colleague Egil Sakshaug for his efforts with this manuscript. We also thank Stephen V. Smith and two anonymous reviewers for their constructive comments and help to improve our manuscript.