Copepod food-quality threshold as a mechanism influencing phytoplankton succession and accumulation of biomass, and secondary productivity: A modeling study with management implications

Development of proactive management schemes may be necessary to combat the apparent worldwide increase in harmful algal blooms. Design of such schemes will require a thorough understanding of bloom-initiating processes in an ecosystem context. To further explore potential synergistic effects between abiotic and biotic processes impacting plankton community dynamics a detailed numerical model was developed and tested. The model featured multiple growth limiting resources (nitrogen, phosphorus, silica, light), multiple phytoplankton groups (P-specialist, N-specialist, intermediate group), aspects of the microbial loop (labile dissolved organic nitrogen, bacteria, microflagellates, ciliates), and a capstone predator (copepods). Model simulations illuminated the potential role of food-quality threshold as it affected initiation of an algal bloom. The mechanism controlling whether a bloom would occur and secondary productivity cease was the timing of the onset of bottom-up control (nutrient limitation) relative to top-down control (high grazing pressure). Simulations where top-down control occurred before bottom-up control were characteristic of Lotka–Volterra type behavior. However, during simulations where top-down control began after bottom-up control an algal bloom resulted and secondary productivity ceased. This occurred because at the time of maximum grazing activity the N-content of one of the phytoplankton groups was below the food-quality threshold for copepods. Consequently, copepod growth was not great enough to offset losses. As a result, the copepod population was eliminated and an algal bloom ensued. The timing of the onset of bottom-up and top-down control was sensitive to some abiotic conditions that included magnitude, mode, and ratio of nutrient loading. Through manipulation of these abiotic processes, it was possible to maintain phytoplankton species diversity, enhance secondary productivity, and prevent an algal bloom.