Relating climatological forcing to coastal water levels in Louisiana estuaries and the potential importance of El Niño-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 w e 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 3 7 % of the intermonthly variabihty and 4 6 % of the Interannual variability in our 25 yr coastal water level and malsh inundation record Coastal water levels explained a small but s i g n ~f ~c a n 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 s p a 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 estuanne flooding reglmes and consequently affects inshore s h n m p 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 '