Abstract
To identify the environmental factors that drive plankton community composition and structure in coastal waters, a shallow northwestern Mediterranean lagoon was monitored from winter to spring in two contrasting years. The campaign was based on high-frequency recordings of hydrological and meteorological parameters and weekly samplings of nutrients and the plankton community. The collected data allowed the construction of correlation networks, which revealed that water temperature was the most important factor governing community composition, structure and succession at different trophic levels, suggesting its ubiquitous food web control. Temperature favoured phytoplanktonic flagellates (Cryptophyceae, Chrysophyceae, and Chlorophyceae) and ciliates during winter and early spring. In contrast, it favoured Bacillariophyceae, dinoflagellates, phytoplankton < 6 µm and aloricate Choreotrichida during spring. The secondary factors were light, which influenced phytoplankton, and wind, which may regulate turbidity and the nutrient supply from land or sediment, thus affecting benthic species such as Nitzschia sp. and Uronema sp. or salinity-tolerant species such as Prorocentrum sp. The central role of temperature in structuring the co-occurrence network suggests that future global warming could deeply modify plankton communities in shallow coastal zones, affecting whole-food web functioning. Environmental forcing factors play a central role in driving plankton community composition and dynamics in marine and freshwater ecosystems. At a global scale, along latitudinal gradients, species distribution and community composition depend on abiotic conditions, such as temperature, light, and nutrients 1. On the other hand, at the local level, food web structure is more affected by biotic processes such as predation, competition, population growth and behaviour 2 , which are constrained by environmental conditions. However, in highly dynamic systems subject to intense environmental stressors, physico-chemical forcing factors may play a predominant role in shaping communities 3,4. Furthermore, the plankton community's response to environmental forcing factors in these systems is challenging to determine, as these factors can be influenced by various elements and are often linked together 5. For example, alongshore wind in coastal ecosystems triggers deep, cool and nutrient-rich water upwelling, thus influencing plankton communities 6,7. Consequently, the conjunction of wind direction and speed, column mixing, water temperature and nutrient concentration changes explains the plankton response during these events. Therefore, it is essential to study multiple environmental parameters together in particularly highly dynamic systems, as they can be tightly linked together. Shallow coastal waters, including coastal lagoons, estuaries, seagrass beds, and coral reefs, are highly dynamic and often exposed to extreme environmental events. Community composition and structure in these zones are driven by environmental forcing factors, mainly because the zones occupy the interface between land and sea 8. These factors influence plankton communities directly or indirectly. For instance, river runoff transports nutrients and terrigenous organic matter, influences water turbidity, and modulates phytoplankton and bacterial production 9,10. Seawater currents or tides recirculate nutrients, which provide critical elements for the food web and enrich local communities of offshore organisms 11-13. These water inputs and precipitation induce significant salinity variations that affect the plankton community 14,15. Moreover, the shallow depth of these coastal waters