Impacts of spatial and temporal variation of water column production and respiration on hypoxia in Narragansett Bay

I begin by summarizing the impacts of climate change on Narragansett Bay (Chapter 1). Many of these changes in climate (increased temperature, increased rainfall, and decreased wind) impact mixing in the water column and thereby may exacerbate hypoxia in Narragansett Bay. In order to examine the effects of metabolism on hypoxia, it was necessary to place metabolic rates into units of oxygen. To accurately convert measurements of carbon to oxygen, I used empirical measurements of production to calculate a photosynthetic quotient (Chapter 2). Specifically, I used concurrent 14C and light bottle oxygen net primary production measurements. The present summer PQ ratio for Narragansett Bay is 1.42 ± 0.09. A survey of spatial and temporal variation in water column metabolism was conducted over two years (2006–2008), in order to provide an annual perspective to metabolism in the summer (Chapter 3). This was particularly important given speculation that primary production has changed over the past several decades as a result of climate change. Spatially, primary production decreased exponentially down the Bay. Respiration was low in the Providence River Estuary, increased in the Upper Bay, and decreased down the West Passage. Based on net 24-hour metabolism, the Providence River was autotrophic, exporting organic matter into the heterotrophic Upper Bay. Production and respiration were comparable in the remainder of the Bay, yielding a net 24-hour metabolism near zero. Seasonally, the highest rates of production, respiration, and net 24-hour metabolism (autotrophy) were in the summer. The overall annual metabolism budget of the Providence River Estuary and the West Passage of the Bay was as follows: total production of 290 gC m−2 y−1 , total respiration of 320 gC m−2 y−1 , and slight net heterotrophy of 30 gC m−2 y −1. Summer metabolism data were examined in conjunction with continuous oxygen sensor measurements in order to examine drivers of hypoxia in Narragansett Bay (Chapter 4). Little hypoxia occurred in summer 2007; summer 2008 had several short hypoxic episodes; and summer 2009 was hypoxic continuously through the latter half of summer. Summers with the most severe hypoxia (2008 & 2009 vs. 2007) had the highest bay-wide volume-weighted production (807 & 857 vs. 402 gO2 m−2 summer−1); respiration did not show the same variation (325 & 289 vs. 324 gO 2 m−2 summer−1). Hypoxia in the Providence River was controlled by stratification. Hypoxia in the well-mixed Upper Bay was caused by elevated respiration, which was fueled by advected organic matter from the Providence River. Hypoxia in the well-mixed, low respiration Mid Bay was caused by advected low oxygen water from Greenwich Bay and the Upper Bay. The Lower Bay had low stratification, respiration, and production, and hypoxia did not occur. Inter-summer variation of hypoxia was due to changes in riverflow driven stratification, water temperature, and wind. To fully understand primary production in the Bay, an analysis was conducted comparing incubation metabolism estimations with estimations from in situ sensors (Chapter 5). In situ monitoring sensors have become increasingly utilized in coastal ecosystems due to their ability to capture highly resolved spatial and temporal dynamics within a system. I created three different algorithms to integrate oxygen fluctuations through night and day in order to calculate metabolism using in situ sensor measurements. By examining multiple methods of both incubations and in situ estimations, we were able to account for differences within in situ and incubation estimations. Incubation production estimations were significantly higher than in situ estimations differing in capturing bloom dynamics. The mechanism behind these differences can be attributed to the exclusion of grazers from bottle samples. Thus, incubation productivity estimations represent a measure of phytoplankton production, whereas in situ productivity estimations represent a measure of system apparent production. In terms of future analysis, the comparison of incubation and in situ metabolism estimations may provide a mechanism to quantitatively examine grazer dynamics. (Abstract shortened by UMI.