Abstract
Zooplankton assemblages in the confined coastal lagoons of La Pletera salt marshes (Baix Ter wetlands, Girona, Spain) are dominated by two species: one calanoid copepod (Eurytemora velox) and the other rotifer (Brachionus gr. plicatilis). They alternate as the dominant species (more than 80% of total zooplankton biomass), with the former being dominant in winter and the latter in summer. Shifts between these taxa are sudden, and intermediate situations usually do not last more than 1 month. Although seasonal shifts between zooplankton dominant species appear to be related with temperature, other factors such as trophic state or oxygen concentration may also play an important role. Shifts between species dominances may be driven by thresholds in these environmental variables. However, according to the alternative stable states theory, under conditions of stable dominance a certain resistance to change may exist, causing that gradual changes might have little effect until a tipping point is reached, at which the reverse change becomes much more difficult. We investigated which are the possible factors causing seasonal zooplankton shifts. We used high-frequency temperature and oxygen data provided by sensors installed in situ to analyse if shifts in zooplankton composition are determined by a threshold in these variables or, on the other hand, some gradual change between stable states occur. Moreover, following the postulates of the alternative stable states theory, we looked at possible hysteresis to analyse if these seasonal zooplankton shifts behave as critical transitions between two different equilibriums. We also examined if top-down or bottom-up trophic interactions affect these zooplankton shifts. Our results show that shifts between dominant zooplankton species in La Pletera salt marshes are asymmetric. The shift to a Eurytemora situation is mainly driven by a decrease in temperature, with a threshold close to 19 °C of daily average temperature, while the shift to Brachionus does not. Usually, the decrease in water temperature is accompanied by a decrease in oxygen oscillation with values always close to 100% oxygen saturation. Moreover, oxygen and temperature values before the shift to calanoids are different from those before the reverse shift to Brachionus, suggesting hysteresis and some resistance to change when a critical transition is approaching. Top-down and bottom-up forces appear to have no significant effect on shifts, since zooplankton biomass was not negatively correlated with fish biomass and was not positively related with chlorophyll, in overall data or within shifts.
Highlights
Physical variability across environmental gradients, biotic effects mediated by top-down or bottom-up forces as well as the trade-offs by constraining traits on species involved determine zooplankton community structure in shallow lakes (Wiggins et al 1980; McQueen et al 1986; Tilman 1987; Wellborn et al 1996; Meerhoff et al 2012; Batzer and Boix 2016; Gascón et al 2016; Tavşanoğlu et al 2017)
Similar alternations among single zooplankton species have been described in permanent water bodies with high fish density, as is the case of La Pletera salt marshes, with seasonal changes between calanoids and Brachionus rotifers, the former dominating in winter, when organic matter concentration is low, and the latter in summer, coinciding with increases in organic matter and with strong oxygen oscillations giving oversaturation during the day and hypoxia during the night (Badosa et al 2007; Cabrera et al 2019; Bas-Silvestre et al 2020)
Two different stable situations alternated in zooplankton composition in La Pletera lagoons: Brachionus dominated in summer and calanoid copepods in winter (Fig. 3A, B)
Summary
Physical variability across environmental gradients, biotic effects mediated by top-down or bottom-up forces as well as the trade-offs by constraining traits on species involved determine zooplankton community structure in shallow lakes (Wiggins et al 1980; McQueen et al 1986; Tilman 1987; Wellborn et al 1996; Meerhoff et al 2012; Batzer and Boix 2016; Gascón et al 2016; Tavşanoğlu et al 2017). Monospecific zooplankton assemblages can be found under extremely high salinities or strong salinity fluctuations in time or space both in inland (Comín et al 1983; Girgin et al 2004; Horváth et al 2014; Tóth et al 2014) or coastal (Attayde and Bozelli 1998; Brucet et al 2010; Emir Akbulut and Tavşanoğlu 2018) brackish or saline waters This is the case of Mediterranean confined coastal lagoons, which are subjected to strong variations in water level, salinity and nutrient concentrations due to the lack of a continuous surface connection with the sea or with the adjacent freshwater habitats (Quintana et al 1998b, 2018; Menció et al 2017; Casamitjana et al 2019). Similar alternations among single zooplankton species have been described in permanent water bodies with high fish density, as is the case of La Pletera salt marshes, with seasonal changes between calanoids (mainly Eurytemora velox) and Brachionus rotifers (almost exclusively a species of the Brachionus plicatilis complex), the former dominating in winter, when organic matter concentration is low, and the latter in summer, coinciding with increases in organic matter and with strong oxygen oscillations giving oversaturation during the day and hypoxia during the night (Badosa et al 2007; Cabrera et al 2019; Bas-Silvestre et al 2020)
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