Planktonic response to pulse or continuous inorganic nutrient inputs: temporal variations and monitoring implications

Abstract

Coastal lagoons are dynamic systems where the mode of nutrient input, continuous or pulsed, or a combination, can significantly affect plankton community structure and function. We conducted a spring field microcosm experiment in a Mediterranean lagoon to evaluate how three nutrient delivery regimes influence planktonic dynamics, using biomass measurements and extracellular enzyme activity (EEA) as functional indicators. Continuous nutrient additions promoted sustained phytoplankton and bacterioplankton growth, indicating bottom-up control and microbial stability. In contrast, pulse treatments saw a brief bacterioplankton bloom followed by delayed phytoplankton increase and a marked decline in zooplankton, suggesting disrupted trophic succession and food quality limitations. The pulse–continuous treatment yielded intermediate responses, confirming that delivery mode, not just nutrient load, modulates ecosystem processes. Elevated β-glucosidase (GLU) and leucine aminopeptidase (LAP) activity under nutrient addition, particularly in continuous treatments, signaled intensified carbon and nitrogen demand and active organic matter mineralization, even when nutrient concentrations appeared low. These findings suggest that, during stable spring conditions, microbial communities rapidly assimilate nutrients, potentially obscuring short-term biogeochemical changes from traditional concentration-based monitoring. Crucially, the timing and frequency of nutrient supply are as influential as total input, with continuous inputs supporting a more stable and efficient microbial loop. This work underscores the ecological importance of recognizing diffuse, low-level nutrient sources, such as groundwater seepage, which may be underrepresented in lagoon management strategies. Integrating functional indicators with delivery mode assessments can improve predictions of eutrophication risks and support more effective conservation planning in shallow coastal systems.

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