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Egg production rates of the copepod Calanus marshallae in relation to seasonal and interannual variations in microplankton biomass and species composition in the coastal upwelling zone off Oregon, USA

Peterson, William T., Du, Xiuning
Progress in oceanography 2015 v.138 pp. 32-44
Bacillariophyceae, Calanus, Ciliophora, Miozoa, biomass, chlorophyll, community structure, egg production, eggs, females, linear models, phytoplankton, seasonal variation, secondary productivity, species diversity, species recruitment, spring, summer, trophic relationships, winter, Oregon
In this study, we assessed trophic interactions between microplankton and copepods by studying the functional response of egg production rates (EPR; eggs female−1day−1) of the copepod Calanus marshallae to variations in microplankton biomass, species composition and community structure. Female C. marshallae and phytoplankton water samples were collected biweekly at an inner-shelf station off Newport, Oregon USA for four years, 2011–2014, during which a total of 1213 female C. marshallae were incubated in 63 experiments. On average, 80% of the females spawned with an overall mean EPR of 30.4. EPRs in spring (Apr–May, average of 40.2) were significantly higher than summer (Jun–Oct; 26.4). EPRs were intermediate in winter (Jan–Feb; 32.5). Interannually, EPRs were significantly higher in 2014 than 2011 and 2012. Total chlorophyll a (Chl a) concentration and diatom abundance both were significantly higher in summer while no seasonal differences were found in abundance of dinoflagellates, ciliates or Cryptophytes. Although total Chl a showed no interannual differences in bulk biomass of phytoplankton, community structure analysis indicated differences among years. More diverse diatom communities were observed in 2013 and 2014 compared to 2011 and 2012. Relationships between EPR and potential food variables (phytoplankton and ciliates) were significant by season: a hyperbolic functional response was found between EPR and total Chl a in winter–spring and summer, separately, and between EPR and ciliate abundance in winter–spring; a linear model fit best the functional response of EPR to diatom abundance in summer. The estimate of potential population recruitment rate (the number of females×EPR; eggs day−1m−2) was highest in spring (Apr–May), and annually was highest in 2013 (11,660), followed by 2011 (6209), 2012 (3172) and 2014 (1480). Our observations of in situ EPR were far higher than published laboratory rates of 23.5, calling into question our past laboratory studies that used mono-algae cultures as food for copepods. Moreover, an increasing number of studies (including this study) are showing an apparent greater importance of ciliates as a nutritious food source for Calanus species demonstrating the importance of the microbial loop to secondary production.