Identification of organic matter sources contributing to hypoxia formation in two eutrophic South Texas estuaries: relationships to watershed land use practices

2014-2016 - $284,735

Dr. Michael Wetz
Department of Life Sciences
Texas A&M University
Corpus Christi

Dr. Xinping Hu
Department of Physical and Environmental Sciences
Texas A&M University
Corpus Christi

Abstract

The main project goal is to identify the main source(s) of organic matter fueling hypoxia in Baffin Bay, Oso Bay and Corpus Christi Bay. Our objectives will be to quantify the lability and degradation rates of organic matter from various sources in each of these systems. Specifically, we will use experimental approaches to quantify the rates of bacterial production and respiration upon allochthonous dissolved organic matter (DOM) and particulate organic matter (POM) from key tributaries of each system, as well as upon phytoplankton-derived POM. Measurements will be made from multiple locations and over the course of two years, providing spatially-temporally explicit understanding of organic matter cycling in these systems. This data will be combined with ongoing water quality monitoring programs that will yield estimates of nutrient and organic matter loadings, as well as spatially-temporally explicit data on nutrients, phytoplankton, organic matter and other important water quality parameters. Specific questions to be addressed include: 1) When, where and under what conditions are symptoms of water quality (i.e., oxygen level, nutrient concentrations) degradation the worst? 2) What are the sources of organic matter fueling water quality degradation? 3) What type of role do the sediments play in regulating bottom water oxygen level, i.e., is there legacy effect of prior nutrient/OM loading and production? We hypothesize that in both Baffin Bay and Oso Bay, phytoplankton-derived organic matter is the dominant source fueling hypoxia due to persistent, excessive phytoplankton biomass throughout much of the year. The second hypothesis would be that certain land use practices (e.g., sewage treatment plants, manure and related fertilization techniques) may lead to localized enrichment of water bodies with labile DOM that promotes hypoxia formation following rainfall events. It is also quite possible that there are multiple co-occurring labile sources of organic matter fueling hypoxia. Third, we hypothesize that pelagic-benthic coupling may also influence oxygen levels as sediments can both consume oxygen through wave pumping and regenerate nutrients through benthic remineralization, the latter is then responsible for secondary primary production in the water column. South Texas coastal embayments are undergoing dramatic changes due to climate change (i.e., prolonged drought), land use change (i.e., increasing prevalence of agricultural or urbanized land uses) and increasing human freshwater needs. Due to the economic, recreational and ecological importance of these embayments, it is imperative to develop ecosystem-based management practices to promote their future viability and ecological health. The powerful complimentary approaches proposed here, including field and experimental techniques, will provide ecosystem-level understanding of the main causes of one of the most obvious and damaging symptoms of eutrophication, hypoxia formation, in three important bay systems of south Texas.