Effects of varying salinity on phytoplankton growth in a low-salinity coastal pond under two nutrient conditions.

Coastal ponds are highly susceptible to negative effects from nu- trient loading (1). The usual approach for managing such systems is to reduce nutrient input. Another possibility for some low-salinity sys- tems may be to control salinity if salinity has a pronounced infl uence 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 fresh- water 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 specifi c 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 mesotro- phic to eutrophic (based on chlorophyll levels; 1), perhaps due to nutrient loading from the expanding residential population sur- rounding the pond. Oyster Pond's salinity has decreased from 32‰ (open to the ocean) to less than 2‰ (road restricting Vineyard Sound infl ow) (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 fi sh populations they desire in the pond (Barry Norris, Oyster Pond Environmental Trust), we are interested in considering what ef- fects salinity changes might have on resident phytoplankton pop- ulations. 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), fi ltered Vine- yard Sound water (GF/F), and deionized water in clear polycar- bonate bottles. The 0.2‰ treatment contained 200 ml Oyster Pond water and 1800 ml deionized water. The 2.3‰ contained 200 ml Oyster Pond water, 129 ml Vineyard Sound water, and 1671 ml deionized water. The 5.0‰ treatment contained 200 ml Oyster Pond water, 298 ml Vineyard Sound water, and 1502 ml deionized water. Three replicate bottles in each salinity treatment were enriched with NaNO3 and NaH2PO4 to fi nal concentrations of 50 M and 3 M, respectively (N P), while another three bottles at each salinity were enriched only with NaH2PO4 to a fi nal concen- tration of 3 M (P). Ambient nitrate and SRP (surface reactive phosphate) concentrations in the pond were 0.2 M and less than 0.5 M, respectively. Since Vineyard Sound water used to set up the 2.3‰ and 5.0‰ salinity treatments contained some nitrate and SRP (0.01 M and less than 0.5 M, respectively), nutrient addi- tions were in excess to avoid a systematic bias. Two mM NaHCO3 was added to each salinity treatment to buffer against CO2 deple- tion and pH changes (3). Bottles were incubated from 24 -29 °C with a 15:9 light:dark cycle. Light intensity ranged from 280 to