Narragansett Bay Hypoxic Event Characteristics Based on Fixed-Site Monitoring Network Time Series: Intermittency, Geographic Distribution, Spatial Synchronicity, and Interannual Variability

Abstract

Low dissolved oxygen events were characterized in Narragansett Bay (NB), a moderate-size (370 km 2 ) 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 O 2 l −1 ) and the deficit-duration (mg O 2 l −1 day; product of deficit and duration [day]). Hypoxia (threshold 2.9 mg O 2 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 O 2 l −1 day. Conditions were more severe to the north and west, a pattern attributed to a northsouth 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 O 2 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 Junemean 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.

Long-Term Trends in Chesapeake Bay Seasonal Hypoxia, Stratification, and Nutrient Loading

“…Thus, the January-May nutrient loads feed a cycle of growth, decay, and recycling that continues throughout the summer, explaining how the duration of summer hypoxia can be correlated with January-May TN loads (Table 6). Similarly, in the Narragansett Bay, which has a much shorter water residence time of 10-40 days, "season-cumulative hypoxia severity" was found to be highly correlated with both the June river flow (which carries nutrients) and June stratification (Codiga et al 2009). …”

Density stratification in an estuary with complex geometry: Driving processes and relationship to hypoxia on monthly to inter‐annual timescales

“…While 3 of the 4 most strongly stratified years (2001,2006,2009) correspond to the 3 highest hypoxia index values, the year with 3rd highest stratification (2003) had hypoxia index equivalent to that during two years when stratification was weaker than average (2002,2008). So although stratification has been shown to be the parameter most closely tied to hypoxia among a wide array of available biological and physical measurements [Codiga et al, 2009], there is a limit to the strength of the relationship. Furthermore, the lack of significant correlations between seasonal hypoxia index and stratification when both are computed at individual stations suggests that local advection, which is not part of the onedimensional paradigm but for which observations are the most limited, is important.…”

Dynamics of wind-driven estuarine-shelf exchange in the Narragansett Bay estuary

“…Hypoxia in bottom waters of the upper estuary typically persists for one to several days (Codiga et al, 2009), similar to time scales of wind fluctuations. Wind-driven exchange events in the lower bay have the potential to relieve hypoxic conditions by advection of oxygenated bottom waters up into the estuary as indicated by excursion lengths 410 km in largest East Passage intrusion events.…”

“…One ecosystem process of concern in Narragansett Bay is summertime hypoxia (Bergondo et al, 2005;Deacutis et al, 2006;Codiga et al, 2009). Hypoxia in estuaries is linked to high nutrient loads that fuel high biological productivity and supply the organic matter necessary for oxygen draw-down (Diaz, 2001;Kemp et al, 2009).…”