Retention time generates short-term phytoplankton blooms in a shallow microtidal subtropical estuary

Abstract

n this study it was hypothesised that increasing water retention time promotes phytoplankton bloomsin the shallow microtidal Patos Lagoon estuary (PLE). This hypothesis was tested using salinity variationas a proxy of water retention time and chlorophyllafor phytoplankton biomass. Submersible sensorsfixed at 5 m depth near the mouth of PLE continuously measured water temperature, salinity and pig-mentsfluorescence (calibrated to chlorophylla) between March 2010 and 12th of December 2011, withsome gaps. Salinity variations were used to separate alternating patterns of outflow of lagoon water(salinity<8; 46% of the time) and inflow of marine water (salinity>24; 35% of the time). The twotransition phases represented a rapid change from lagoon water outflow to marine water inflow and amore gradually declining salinity between the dominating inflow and outflow conditions. During thelatter of these, a significant chlorophyllaincrease relative to that expected from a linear mixing rela-tionship was observed at intermediate salinities (10e20). The increase in chlorophyllawas positivelyrelated to the duration of the prior coastal water inflow in the PLE. Moreover, chlorophyllaincrease wassignificantly higher during austral spring-summer than autumn-winter, probably due to higher light andnutrient availability in the former. Moreover, the retention time process operating on time scales of daysinfluences the long-term phytoplankton variability in this ecosystem. Comparing these results withmonthly data from a nearby long-term water quality monitoring station (1993e2011) support the hy-pothesis that chlorophyllaaccumulations occur after marine inflow events, whereas phytoplankton doesnot accumulate during high water outflow, when the water residence time is short. These results suggestthat changing hydrological pattern is the most important mechanism underlying phytoplankton bloomsin the PLE.©2015 Elsevier Ltd. All rights reserved.1. IntroductionPhytoplankton variability in coastal ecosystems is determined bydiverse range of factors and complex interacting site-specific pro-cesses (Cloern, 2001; Cloern and Jassby, 2010; Gallegos et al., in thisissue). Differences among ecosystems may result from physical,geomorphological and hydrodynamic characteristics, but alsonutrient enrichment, climatology and human disturbances. In manyestuaries and coastal lagoons, phytoplankton biomass and speciescomposition variability are strongly associated with hydrodynamics,when basic growth requirements (light and nutrients) are plenty(Peierls et al., 2012; Thompson et al., in this issue).Coastal lagoons are shallow, dynamic and highly productiveecosystems separated from the ocean by a sand barrier that ispenetrated by one or several channels allowing water exchangewith the ocean. Coastal lagoons are classified as choked, restrictedor leaky according to their degree of water exchange with the ocean(Kjerfve, 1986). Due to the restricted water exchange, choked la-goons have comparatively long water retention time and highphytoplankton biomass (Knoppers et al., 1991; Roselli et al., 2013).The Patos Lagoon in Southern Brazil (Fig. 1), is the largest(10,360 km2) choked coastal lagoon in the world (Kjerfve, 1986).The connection with the coastal ocean in the southern area of thePatos Lagoon has typical estuarine conditions, which influence thegrowth and distribution of all biota from primary producers tofishes (Seeliger et al., 1997; Odebrecht et al., 2010). In the Patos*Corresponding author. Department of Bioscience, Aarhus University, Freder-iksborgvej 399, 4000 Roskilde, Denmark.E-mail address:jac@dmu.dk(J. Carstensen).Contents lists available atScienceDirectEstuarine, Coastal and Shelf Sciencejournal homepage:www.elsevier.com/locate/ecsshttp://dx.doi.org/10.1016/j.ecss.2015.03.0040272-7714/©2015 Elsevier Ltd. All rights reserved.Estuarine, Coastal and Shelf Science 162 (2015) 35e44