Abstract
Relative sea-level rise is resulting in the intrusion of saline waters into marshes historically dominated by fresh water. Saltwater intrusions can potentially affect resident marsh species, especially when storm-related tidal surges cause rapid changes in salinity. We examined the role of historical salinity exposure on the survival of Gambusia affinis from two locations in coastal Louisiana. At each location, we sampled fish populations from fresh, intermediate and brackish marshes. Individuals were then exposed to a salinity of 25‰ and survival time was measured. We found that fish from brackish and intermediate marshes had an increased tolerance to salinity stress relative to fish from freshwater environments. We then tested the descendents of fish from the fresh and brackish marshes, reared for two generation in fresh water, to determine if there was a genetic basis for differential survival. We found that descendents of individuals from brackish marshes showed elevated survivals relative to the descendents of fish with no historical exposure to salinity. The most reasonable mechanism to account for the differences in survival relative to historical exposure is genetic adaptation, suggesting that natural selection may play a role in the responses of resident marsh fishes to future increases in salinity.
Highlights
The coastal marshes of Louisiana are experiencing sea-level rise at rates of approximately 10–11 mm/year; these rates are 10 times the global averages (Penland and Ramsey 1990; Gornitz 1995)
Fish collected from brackish marsh habitats had significantly greater survival than those collected from habitats without historical salinity exposure when treated with a salinity stress of 25&
The wide range of salinity tolerance known to occur in marsh residents like Gambusia (Al-Daham and Bhatti 1977; Chervinski 1983; Meffe and Snelson 1989) is influenced strongly by historical salinity exposure
Summary
The coastal marshes of Louisiana are experiencing sea-level rise at rates of approximately 10–11 mm/year; these rates are 10 times the global averages (Penland and Ramsey 1990; Gornitz 1995). During the last 50 years, Louisiana has lost coastal lands at a rate of over 130 km per year This results in stands of marsh vegetation separated by open water (Sasser et al 1986; Rozas and Reed 1993). As marsh areas are converted to shallow open water areas, saline waters can penetrate farther up the estuary because there is less resistance from the marsh to up-estuary flows from tidal flux and storm surge. Fish inhabiting these deteriorating coastal marshes may be especially susceptible to salinity stress when hurricanes and other storms push salt water into lower salinity marshes. Fish living in brackish conditions may acquire genetic adaptations through natural selection for higher individual salinity tolerance than fish from freshwater environments where there has been no selection for salinity tolerance
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