Potential growth rates of phytoplankton biomass were estimated year-round from production rates and biomass and were compared with observed changes in euphotic phytoplankton biomass. Potential growth was always greater than observed growth. The discrepancy between both is attributed to losses. Relative loss rates showed wide seasonal fluctuations with highest values during the spring bloom and autumnal phytoplankton maximum, respectively. Loss rates of photoassimilated carbon showed one peak in late May which lead to a clear-water phase. Relative loss rates were highly correlated with potential growth rates whereas observed growth rates were not. This suggests that most losses occur immediately after the production process and do not lead to increases in biomass. During the spring bloom grazing by zooplankton is the single most important factor leading to losses from the phytoplankton community. During that time, 80–98% of overall losses can be accounted for by grazing, sedimentation and wash-out combined. During brief periods in summer and autumn, sedimentation rates comprised >50% of overall losses. In autumn only 30–40% of overall losses were due to the above-mentioned processes. Residual losses can be attributed to respiration, lysis and bacterial remineralization. Grazing, respiration and lysis lead to recycling of carbon and nutrients. Sedimentation rate measurements suggest an average euphotic carbon regeneration rate of 85%. For the transfer efficiency of carbon along the food chain the relative significance of respiratory losses in overall losses is of fundamental importance.