The salt content (salinity) of water in the San Francisco Estuary can vary significantly. Since the estuary connects to the ocean it is tidally influenced and saltwater can flow into the estuary. How far this oceanic saltwater intrudes into the estuary can vary by tidal cycles as well as the amount of freshwater flow the Sacramento and San Joaquin rivers are pouring into the estuary. During spring tides high salinity water can penetrate further into the estuary, while neap tides tend to infiltrate less so. When there is significant freshwater flow out of the delta it can push the high salinity water back oceanward, creating more low salinity habitat in the estuary.
Estuarine communities are evolved to withstand changing salinities, but some species have different tolerances of how salty of water they can withstand. Similarly, some species can withstand high salinity as adults, but can not at earlier life stages, or vice versa. High salinity waters can interfere with osmoregulation in some organisms. Osmoregulation is the process by which organisms manage water and ion concentrations inside their bodies for proper cellular function, and an inability to properly regulate these concentrations can be lethal. Additionally, salinity can influence local ecology, potentially changing algae and zooplankton communities and abundances.
The majority of fish in the world are either freshwater or saltwater species. Freshwater fish species cannot survive in salt water, likewise, saltwater species cannot survive in freshwater. However, as always, in the world of fish there are exceptions. Longfin smelt are one of the exceptional species that can transition between saltwater and freshwater. This is an impressive biological feat where individuals practically reverse the function of their cells to excrete salts as opposed to actively up taking them in freshwater. As mentioned in previous blog posts, longfin smelt are anadromous, meaning they live in coastal and high salinity waters as adults, then migrate up the estuary into fresher water to spawn. It is in these low salinity waters that the larvae hatch, feed, and grow.
The amount of saltwater in the estuary is actively managed at times in order to keep saline water away from water pumps, and to maintain low salinity habitat for several threatened and endangered species in the estuary. They can manage the salinity of the water in the estuary by releasing freshwater from reservoirs that flows down the rivers and pushes the high salinity water out of the estuary. Using estimates where in the estuary the salt concentration is 2 parts per thousand (ppt) on any given day managers have a general idea of where the low salinity waters exist.
Longfin smelt larvae are susceptible to high salinities and can be precluded from making it into adulthood if they hatch in waters that are too saline. It is thought that 2 ppt is the ideal concentration for successful recruitment of larvae into the adult population, and that recruitment becomes rarer at concentration greater than 6 ppt (Hobbs et al. 2010).
Considering longfin smelt may rely on salinity concentrations as a spawning cue, and knowing that salinity concentrations can affect longfin survival, it is important we examine how salinity is affecting longfin smelt annually. Using data gathered from three water quality monitoring stations in the estuary I will be compiling a dataset that summarizes salinity conditions annually. I will be examining multiple concentrations that researchers have suggested are important to growth and survival. Using these data we will hopefully be able to better inform our growth and abundance models of this species.
Picture: Jones Pumping Plant near Tracy, California. Taken by John Ridilla.
Hobbs, James A., et al. "The use of otolith strontium isotopes (87 Sr/86 Sr) to identify nursery habitat for a threatened estuarine fish." Environmental biology of fishes 89.3 (2010): 557-569.
Agency: U.S. Fish and Wildlife Service
Program: US Fish & Wildlife Service - DFP
Location: San Francisco Bay National Wildlife Refuge Complex
