Preliminary Results

1. Foodweb support for the threatened Delta smelt: Salinity effects on phytoplankton and dissolved organic carbon availability Risa A. Cohen1, Anne M. Slaughter2, Edward J. Carpenter2 1 Department of Biology, Georgia Southern University, Statesboro, GA 2 Romberg Tiburon Center for Environmental Studies, San Francisco State University, Tiburon, CA 0 20 San Francisco Bay and Delta Kilometers Sacramento River CA San Pablo Bay Suisun Bay Carquinez Strait San Joaquin River sampling area 0.5-5 psu Mar-Aug 2006 San Francisco Scenedesmus spp. Skeletonema spp. Skeletonema spp.sfbay.wr.usgs.gov/access/Cole Cloern/Necklace.html Scenedesmus spp. Pseudodiaptomus forbesi www.fish.washington.edu/ people/cordell//gallery.html Delta Smelt www.fws.gov/pacific/ecoservices/envicon/pim/reports/Sacramento/SacramentoDelta.htm Introduction Preliminary Results Conclusions and Implications Skeletonema spp. saline Scenedesmus spp. fresh This study is part of a collaborative program to characterize the foodweb of the low salinity zone (LSZ) of the northern San Francisco Estuary (SFE). Recent evidence indicates that estuarine fish, including Delta Smelt, may be food limited, suggesting a link between their declines and changes at lower trophic levels. Phytoplankton production is low, but is the dominant source of • The effects of salinity on phytoplankton depended on their origin (fresh vs. more saline water) • Skeletonema spp. cells were adversely affected when exposed to low salinity water: lower carbon fixation, lower chlorophyll concentration and poorer cell condition at lower salinity • In contrast, Scenedesmus spp. cells were apparently not affected when exposed to higher salinity water: similar carbon fixation, chlorophyll concentration and cell condition across all salinity treatments; this result suggests tolerance to short-term increases in ambient salinity • Increased water column nutrient concentrations may mitigate the effects of increased salinity for freshwater phytoplankton by serving as osmolytes upon uptake. We did not observe a nutrient effect on Scenedesmus spp., likely due to its tolerance of higher salinities • It appears from these data that phytoplankton from tidal origin may contribute DOC following lysis under low salinity conditions • DOC concentrations from these experiments (awaiting analyses) will be necessary to establish whether DOC in this system comes from saline or freshwater phytoplankton, or both No nutrients 5 psu 2 psu 0.5 psu Carbon fixation (slope) was similar across all salinity treatments, with or without nutrient addition, suggesting Scenedesmus spp. cells were not stressed when exposed to higher salinities. The DO data (not shown) supported this finding. Cells at the highest salinity had the highest carbon fixation rates (slope), suggesting low salinity conditions are stressful. This result was supported by DO data (not shown). A slight increase in C fixation with added nutrients was evident. DIC (mg*L-1) + Nutrients 5 psu 2 psu 0.5 psu Time (hours) particulate and dissolved organic carbon (POC and DOC) to the base of the foodweb in the LSZ (Sobczak et al. 2002). Salinities in the LSZ may be partly responsible for low phytoplankton biomass. Brackish salinities are high enough to damage freshwater phytoplankton and low enough to lyse phytoplankton adapted to more saline tidal water. Degraded phytoplankton may release DOC to the water column supporting bacterial production, rather than being directly consumed by zooplankton. Cells in the highest salinity treatment had the highest chlorophyll levels by the end of the 12 h experiment, suggesting Skeletonema spp. cells ruptured in lower salinity treatments. Addition of nutrients did not change this effect. By the end of the 12 h experiment, Scenedesmus spp. cells in all three salinity treatments had equivalent amounts of chlorophyll regardless of nutrient addition. Chlorophyll a (µg*L-1) Objectives To assess whether 1) salinity in the LSZ results in rapid degradation of phytoplankton cells and increased release of DOC and 2) nutrient availability mitigates salinity effects on freshwater phytoplankton. Literature Cited Time (hours) Sobczak, W. et al. 2002. Bioavailability of organic matter in a highly disturbed estuary. PNAS, 99: 8101-8110. Microscopic examination of cells exposed to lower salinities confirmed lysis. The lowest salinity treatment had the highest proportion of dead cells and “blown out” cells were clearly visible. While some cell shrinkage was observed in Scenedesmus spp. exposed to higher salinities, cell numberswere similar across all treatments and did not necessarily lead to cell death. Acknowledgments Materials and Methods We wish to thank Captain David Morgan and David Bell for their assistance aboard R/V Questuary. Special thanks to Ulrika Lidstrom for field collections and cell counts, Al Marchi for water nutrient analyses and Chris Ikeda for assistance with experiments. Funding for this project was provided by CALFED Science Program Grant # SCI-05-C107. • Collected water from 3 different salinity zones (0.5, 2 and 5 psu) in the LSZ to culture phytoplankton assemblages for salinity tolerance experiments; data from Scenedesmus spp. (from 0.5 psu) and Skeletonema spp. (from 5 psu) experiments are presented • Exposed known concentrations of phytoplankton to 0.5, 2 and 5 psu water, with or without nutrient additions, for 12 h • Sampled phytoplankton and water at 0, 3, 6 and 12 h to determine dissolved inorganic carbon (DIC), chlorophyll a, DOC and dissolved oxygen and nutrient concentrations • Preserved cells for microscopic assessment of cell health and concentration Percentage of dead/damaged cells (%) Further Information Email: rcohen@georgiasouthern.edu, aslaught@sfsu.edu Time (hours)