Relating climatological forcing to coastal water levels in Louisiana estuaries and the potential importance of El Nino-Southern Oscillation events

Between summer 1987 and winter 1988 coastal water levels and marsh inundat~on regimes in the Mississippi h v e r Deltaic Plaln of Louis~ana (USA) were anomalously low This penod of low water levels colresponded with the 1987-88 La Nina event, and we hypothesized that the low estuanne water levels were a local manifestation of large-scale cl~matological forcing related to thls unusual event We used 25 yr (1963 to 1988) of hydrological, climatological and biological data to investigate this possible relationship between El Nino-Southern Osclllat~on (ENSO) and La Nina events and water levels In Lou~siana estuaries Records of continuous watel levels at brackish and saline marsh sites in the Barataria Basin estuary Louisiana from 1987 to 1989 showed the extended p e r ~ o d of low marsh inundation We used a 25 yr water level record (1963 to 1988) from a nearby gauge to ascertain the historical frequency of this type of phenomenon Our site-specific marsh flooding data were correlated w t h corresponding 1987-88 water levels from this long-term data set confirming that historical water level data adequately represented histor~cal marsh inundation Water levels In Louisiana s m~crotldal estuaries are closely tied to climatological forcing, and multivanate analysis indicated that wind d ~ r e c t ~ o n local precipitation and temperature and Mississippi h v e r discharge together significantly explained 37% of the intermonthly variabihty and 46% of the Interannual variability in our 25 yr coastal water level and malsh inundation record Coastal water levels explained a small but s ign~f~can t portion (rL = 0 26) of the observed interannual vanabllity (1963 to 1988) In inshore harvest of shrimp a fishery dependent on seasonal access to estuarine marshes The relat~onship between water levels (marsh inundation) and shnmp harvest was curvilinear shnmp catches were low at both low and hlgh coastal water levels while highest shrlmp product~vlty occurred In years with intermediate water levels Lowest shrimp catches generally coincided with ENSO and La Nina events The 1987-88 La Nliia event, for example coincided w ~ t h an anomalously low marsh inundation reglme that functionally isolated marshes from the estuanne water column for long periods This event may have thus negat~vely impacted shrimp harvests by denying juveniles access to intertidal marsh habitat IVhen ENSO events coincided w ~ t h anomalously h ~ g h marsh Inundation reglmes high water levels were generally associated with high rates of local precipitation and Mississippi h v e r discharges Although marshes were accessible durlng these events spa t~a l reduction of h ~ g h e r salinity waters by large fresh water Inputs may have decreased juvenile shrimp habitat and thus contnbuted to reduced s h r ~ m p harvests In conclusion, we found that climatological forcing has direct effects on es tuanne flooding reglmes and consequently affects inshore shnmp harvests In coastal Louisiana Furthermore, the nonlinear relationship between shrimp catch and water level is a result of low-catch endpoints that often correspond to ENSO and La N ~ n a events