Patterns of microzooplankton growth in dilution experiments across a trophic gradient: Implications for herbivory studies
To investigate the growth and grazing patterns of microzooplankton (MZP) in environments of differing productivity, dilution experiments measuring phytoplankton growth (μ) and grazing mortality (m) rates were performed using samples from contrasting locations along the Texas coast. Samples were collected from estuaries, coastal lagoons and offshore Gulf of Mexico locations in the spring and summer of 2001. MZP growth rates were determined in each dilution treatment. Although MZP biomass changed over time in most dilution treatments, adjusting μ and m for the actual grazer gradient (represented by geometric mean MZP biomass) did not cause a significant deviation from the nominal dilution gradient. Likewise, these adjustments did not yield significant regressions where none existed before adjustment. The dynamics of MZP taxonomic groups (ciliates, dinoflagellates) and size categories differed suggesting that in some cases internal predation may lead to trophic cascades. MZP biomass was higher in productive coastal waters and included a larger proportion of dinoflagellates than in the oligotrophic, ciliate-dominated waters of the Gulf of Mexico. The MZP biomass-to-chlorophyll a ratio was lowest in the hypereutrophic Nueces River, where MZP biomass significantly increased in all dilution treatments (net growth rates up to 2 day−1) suggesting a strong top–down control. In the brown-tide dominated Upper Laguna Madre and the oligotrophic seagrass-dominated Lower Laguna Madre MZP growth was decoupled from that of phytoplankton. At these sites, MZP were likely fueled by bacterial carbon and mixotrophy, respectively. Observing the growth response of MZP in dilution experiments can provide insight into trophic structure and efficiency of the microbial food web.
KeywordsPhytoplankton Dinoflagellate Phytoplankton Growth Grazing Rate Grazing Impact
The University of Akron Graduate Program and the Nancy Lee and Perry Bass Endowed Chair funding to the University of Texas at Austin Marine Science Institute (UTMSI) supported this work. W.S. Gardner and M.J. McCarthy generously assisted with logistics, sampling and analyses. We are extremely grateful for their support. MRF also received support from the University of Georgia Graduate Fellowship. We thank K.H. Dunton and his students who shared space on the Laguna Madre cruise and the personnel at UTMSI and captain and crew of R/V Longhorn who assisted with this work. R.J. Duff and L. Fraser assisted much with the preparation of the thesis on which this work is based. A.M. Modugno was generous in her support and editorial skills. The comments and suggestions of three anonymous reviewers greatly improved this manuscript.
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