Narragansett Bay Hypoxic Event Characteristics Based on Fixed-Site Monitoring Network Time Series: Intermittency,Geographic Distribution,Spatial Synchronicity,and Interannual Variability |
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| Authors: | Daniel L Codiga Heather E Stoffel Christopher F Deacutis Susan Kiernan Candace A Oviatt |
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| Institution: | (1) Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, USA;(2) Narragansett Bay National Estuary Program, University of Rhode Island, Box 27, Bay Campus, Narragansett, RI 02882, USA;(3) Department of Environmental Management, Office of Water Resources, 235 Promenade Street, Providence, RI 02908, USA |
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| Abstract: | Low dissolved oxygen events were characterized in Narragansett Bay (NB), a moderate-size (370 km2) temperate estuary with a complex passage/embayment geometry, using time series from 2001 to 2006 at nine fixed-site monitoring
stations. Metrics for event intensity and severity were the event-mean deficit relative to a threshold (mg O2 l−1) and the deficit-duration (mg O2 l−1 day; product of deficit and duration day]). Hypoxia (threshold 2.9 mg O2 l−1) typically occurred intermittently from late June through August at most stations, as multiple (two to five per season) events
each 2 to 7 days long with deficit-duration 2 to 5 mg O2 l−1 day. Conditions were more severe to the north and west, a pattern attributed to a north–south nutrient/productivity gradient
and east–west structure of residual circulation. Spatial patterns for suboxic and severely hypoxic events (thresholds 4.8
and 1.4 mg O2 l−1) were similar. The view that different processes govern event variability in different regions, each influenced by local
hydrodynamics, is supported by both weak spatial synchronicity (quantified using overlap of event times at different sites)
and multiple linear regressions of biological and physical parameters against event severity. Interannual changes were prominent
and season-cumulative hypoxia severity correlated with June-mean river runoff and June-mean stratification. Benthic ecological
implications for areas experiencing events include: NB hypoxia classifies as periodic/episodic on a near-annual basis; highest
direct mortality risk is to sensitive and moderately sensitive sessile species in the northern West Passage and western Greenwich
Bay, with some risk to Upper Bay; direct risk to mobile species may be ameliorated by weak spatial synchronicity; and indirect
impacts, including reduced growth rates and shifts in predator–prey balances, are very likely throughout the sampled area
due to observed suboxic and hypoxic conditions. |
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