Modeling the formation of periodic hypoxia in partially mixed estuaries and its response to oligotrophication and climate change

• Main Author: Lake, Samuel J.
• Format: Others
• Language: English
• Published: W&M ScholarWorks 2013
• Subjects: Climate; Ecology and Evolutionary Biology; Environmental Sciences; Marine Biology
• Online Access: https://scholarworks.wm.edu/etd/1539616727; https://scholarworks.wm.edu/cgi/viewcontent.cgi?article=2294&context=etd

The development of hypoxia represents one of the most common and ecologically detrimental effects of anthropogenic nutrient enrichment in coastal marine ecosystems. Due to the physiological importance of oxygen as a key component of metabolic processes, the development and persistence of hypoxia can reduce the distribution of important species, modify food webs, decrease diversity and richness, and sub-lethally affect growth and reproductive rates. While many recent studies have focused on the global increase in hypoxia and highlighted the need for nutrient reduction strategies, some key processes associated with hypoxia remain understudied. of particular importance is the resolution of the major carbon sources fueling hypoxia in tributary estuaries, which receive inputs from the upland watershed, internal primary production, and advection from the main estuary, which may also be a source of hypoxic water. Development of well-constrained, intermediate complexity ecosystem models is also needed to provide realistic predictions of the response of hypoxia to nutrient reduction strategies, and to understand the interactive effects of these load reductions with ongoing climate change. The recent implementation of high spatial and temporal resolution water quality sampling instruments has confirmed the importance of the spring-neap tidal cycle and its effect on the formation and disruption of stratification and hypoxia in the York River estuary (YRE). However, these results have indicated that the advection of high-salinity hypoxic water into the lower YRE from the Chesapeake Bay (CB) may be as important as internal oxygen consumption. Additionally, previous studies have suggested that phytoplankton production in the YRE and similar tributary estuaries may be insufficient to explain the magnitude of hypoxia observed. This study synthesized in-situ measurements and high resolution water quality monitoring with an intermediate complexity model to examine the significance of these factors and how they interact to cause hypoxia within the YRE. Simulations were used to determine the magnitude of nutrient and/or organic matter (OM) reductions required to reduce the extent and severity of hypoxia in the presence of increasing temperatures resulting from climate change. A comparison of in-situ and computed oxygen concentrations for the YRE indicated that internal respiration was sufficient to drive hypoxia under stratified conditions, without the need for advection of hypoxic water from the CB. Phytoplankton production was the major source of organic carbon to the YRE and 1.5 times greater than advective inputs from CB, which were roughly balanced by exports. Watershed sources and microphytobenthos contributed comparatively little carbon to the whole system. Model simulations indicated that lower portions of the YRE tributaries are strongly influenced by watershed OM loading during summer, while the low mesohaline region is influenced by internal primary production and OM from the tributaries. The high meso- and polyhaline regions responded primarily to advected dissolved organic carbon from CB. Results indicate that different regions of the YRE require separate management strategies to control hypoxia, with the key issue in the lower estuary being the "far field" source of labile OM from outside the system. The model predicted increasing primary production under warmer conditions in winter and spring throughout most of the YRE, but decreasing production in summer and fall in the lower estuary. These changes together with increasing respiration resulted in increased autotrophy in the upper YRE, while NEM was predicted to decrease throughout the rest of the estuary. Warmer temperatures increased both the temporal and spatial extent of hypoxia in the model, suggesting the need for additional nutrient and OM load reductions in order to achieve the same level of improvement predicted without warming.