LAUSR

To appreciate coastal trophodynamics, it is necessary to understand the dynamics and control of the spring and late summer/autumn phytoplankton blooms. Classically mesozooplankton are considered as main players in these blooms. Microzooplankton likely also are important in these dynamics, but their role is poorly understood. Critically, due to their rapid generation times, microzooplankton may exhibit rapid shifts during blooms. Through field sampling and rate measurements (dilution experiments) in a well-studied temperate coastal ecosystem (Helgoland, southern North Sea) we ask if there are differences in the trends exhibited between and within the spring and late summer/autumn blooms. To achieve this, we examined early, mid and late bloom periods in both seasons. We found 1) a shift in trophic composition during both blooms, with a trend from strongly autotrophic mixotrophs (e.g. Mesodinium ) to mixotrophs and then towards heterotrophs; 2) an increase in intraguild predation at the end of the blooms; and 3) although microzooplankton were major consumers of the spring bloom (grazing coefficient g : 0.23–0.25 d −1 ; daily percent loss of production P p : 36–47%), they were unlikely to control it, while in contrast, microzooplankton appeared to play a major role in controlling the late summer/autumn bloom (grazing coefficient g : 0.14–1.53 d −1 ; daily percent loss of production P p : 24–103%). In doing so, we suggest that any simplifications that consider these seasonal blooms to be relatively homogeneous and similar will lead to substantial errors in the assessment of coastal trophodynamics.