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Severe Droughts Reduce Estuarine Primary Productivity with Cascading Effects on Higher Trophic Levels
Citation:
Wetz, M. S., E. A. Hutchinson, R. S. LUNETTA, H. W. Paerl, AND C. J. Taylor. Severe Droughts Reduce Estuarine Primary Productivity with Cascading Effects on Higher Trophic Levels. LIMNOLOGY AND OCEANOGRAPHY. American Society of Limnology and Oceanography, Lawrence, KS, 56(2):627-638, (2011).
Impact/Purpose:
Estuaries are among the most productive and diverse aquatic ecosystems on Earth, providing food resources and habitat for many ecologically and economically important fish and shellfish species (Hobbie 2000; Bianchi 2007). Adequate freshwater delivery is vital to support the biodiversity, productivity and fishery habitat of estuaries (Nixon and Buckley 2002). Estuaries also process terrigenous material transported by rivers and play a major role in regional and global biogeochemical cycles (Bianchi 2007). Over the past century, natural cycles of freshwater delivery to the coastal zone have been altered due to human activity and climate change (Milliman et al. 2008). There is increasing concern that ongoing anthropogenic nutrient enrichment, human modification of hydrologic regimes, and climate change will drive estuaries to an unsustainable status through negative effects on water quality and habitat (Flemer and Champ 2006). One particularly worrisome climate change scenario centers on droughts, which are predicted to increase in frequency and severity over the coming century in many world regions (Trenberth et al. 2003; Christensen et al. 2007). Through their potential to dramatically reduce freshwater delivery to estuaries, droughts represent an important driver of contemporaneous and future estuarine ecosystem functioning. Nonetheless, lack of sufficiently long time-series has left the scientific community with only a limited understanding of the full ecological effect that these climatic stressors have on estuaries and the coastal zone.
Description:
Using a 10 year time-series dataset, we analyzed the effects of two severe droughts on water quality and ecosystem processes in a temperate, eutrophic estuary (Neuse River Estuary, North Carolina). During the droughts, dissolved inorganic nitrogen concentrations were on average 46-68% lower than the long-term mean due to reduced riverine input. Phytoplankton productivity and biomass were slightly below average for most of the estuary during a spring-fall drought in 2002, but were dramatically lower than average throughout the estuary during a fall-winter drought in 2007-2008. Droughts affected upper trophic levels through alteration of both habitat condition (i.e., bottom water dissolved oxygen levels) and food availability. Bottom water dissolved oxygen levels were near or slightly above average during the 2002 drought and during summer 2007. Concomitant with these modest improvements in bottom water oxygen condition, fish kills were greatly reduced relative to the long-term average. Low oxygen bottom water conditions were more pronounced during summer 2008 in the latter stages of the 2007-2008 drought, and mesozooplankton bundances were 8-fold lower in summer 2008 than during non drought years. Below average mesozooplankton abundances persisted for well over one year beyond cessation of the drought. Significant fish kills were observed in summer 2008 and 2009, perhaps due to the synergistic effects of hypoxia and reduced food availability. These results indicate that droughts can exert both ephemeral and prolonged multi-year influence on estuarine ecosystem processes and provide a glimpse into the future, when many regions of the world are predicted to face increased drought frequency and severity due to climate change.