Pigment-specific rates of phytoplankton growth and microzooplankton grazing in a subtropical lagoon

Phytoplankton growth and microzooplankton grazing rates were evaluated in one station in Bahía Concepción, located in the middle region of the Gulf of California, México. We used high-performance liquid chromatography (HPLC) estimations of phytoplankton pigment signatures to evaluate the annual variation of taxon-specific grazing and growth rates obtained with the dilution technique. Chlorophyll-a (Chl-a) concentrations varied widely (0.34-3.32 μg L^-1) and showed two maxima, during late spring and autumn, associated with the transition between mixed and stratified conditions. Phytoplankton growth rates varied seasonally with the lowest rates during summer (range: 0.01-2.55 day^-1 for Chl-a; 0.00-3.84 day^-1 for Chl-b; 0.26-3.29 day^-1 for fucoxanthin; 0.00-6.27 day^-1 for peridinin; 0.00-4.35 day ^-1 for zeaxanthin). Microzooplankton grazing was an important loss process (range: 0.0-1.89 day^-1 for Chl-a; 0.00-3.12 day^-1 for Chl-b; 0.26-3.29 day^-1 for fucoxanthin; 0.00-2.03 day ^-1 for peridinin; 0.00-3.51 day^-1 for zeaxanthin). Average grazing rates accounted ∼68-89% of estimated average phytoplankton pigment-specific growth rates. The analysis of pigment signatures indicates that diatoms and dinoflagellates were the dominant groups, and contrary to expectation for typical subtropical lagoons, the specific growth rates in Bahía Concepción showed a pronounced seasonal variability, linked to transitional hydrographic conditions. Our results indicate a close coupling between the community microzooplankton grazing and phytoplankton growth rates, without selective feeding behavior. These results suggest that microzooplankton play a critical role and may significantly modify the availability and efficiency of transfer of energy to higher trophic levels.