Effects of Varying Salinity on Phytoplankton Growth in a Low-Salinity Coastal Pond Under Two Nutrient Conditions

Abstract

Coastal ponds are highly susceptible to negative effects from nutrient loading (1). The usual approach for managing such systems is to reduce nutrient input. Another possibility for some low-salinity systems may be to control salinity if salinity has a pronounced influence on phytoplankton growth. Freshwater species generally compose the phytoplankton of low-salinity systems. One might expect growth to slow as salinity increases until the assemblage switches from freshwater to marine. Similarly, phytoplankton native to systems with fairly constant salinity through space and time may not tolerate any change in salinity, as they may be adapted to that specific salinity (Valiela, Boston University, pers. comm.). Oyster Pond (Falmouth, MA) is a brackish pond connected to Vineyard Sound through a lagoon. The pond is currently mesotrophic to eutrophic (based on chlorophyll levels; 1), perhaps due to nutrient loading from the expanding residential population surrounding the pond. Oyster Pond's salinity has decreased from 32‰ (open to the ocean) to less than 2‰ (road restricting Vineyard Sound inflow) (2). Currently, dredging and a weir maintain the salinity at a fairly constant 2.3‰. Oyster Pond managers have the option of manipulating salinity within the pond via the weir. While managers plan to manipulate salinity according to which fish populations they desire in the pond (Barry Norris, Oyster Pond Environmental Trust), we are interested in considering what effects salinity changes might have on resident phytoplankton populations. To determine if the general Oyster Pond phytoplankton population could adapt to changes in salinity, we added excess nutrients (nitrate and phosphate) under three salinity regimes. To determine if cyanobacteria could adapt to changes in salinity under N-depleted conditions, we added excess phosphate. Water was collected from the northern end of Oyster Pond. Three salinity treatments (0.2‰, 2.3‰, and 5.0‰) under two nutrient conditions were created by mixing sieved Oyster Pond water (150-m mesh to remove macrozooplankton), filtered Vineyard Sound water (GF/F), and deionized water in clear polycarbonate bottles. The 0.2‰ treatment

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