Urban Estuaries Defy Expectations by Weathering Storms Better Than Pristine Ones

When a major storm barrels toward a coastline, scientists and coastal managers usually assume the worst for the estuaries downstream of cities. Urban watersheds shed rain quickly, scour streambeds, deliver pulses of nitrogen and sediment, and generally make aquatic ecosystems less stable. But a new continental-scale study of five American estuaries has upended that intuition: the most urbanized estuaries in the dataset were, on average, the most resistant to precipitation events, holding their dissolved oxygen levels steadier through storms than their more pristine counterparts. The finding, published in the journal Biogeosciences, carries uncomfortable implications for how we measure ecosystem health and how we plan for a future of heavier rainfall and expanding cities.

The research team, led by Anna B. Tureţcaia and Emily B. Graham of Pacific Northwest National Laboratory, together with colleagues from the National Estuarine Research Reserve System, the University of Wisconsin–Madison, and San Francisco State University, set out to answer a deceptively simple question: what makes an estuary resist a storm? They defined resistance as the magnitude of ecosystem change induced by a precipitation event, focusing on dissolved oxygen as an integrative indicator of estuarine function. Dissolved oxygen is the currency of aquatic life, sustaining fish, invertebrates, and the microbial communities that drive carbon and nutrient cycling, and it responds rapidly to shifts in temperature, salinity, nutrient loading, and water movement. A system that maintains stable oxygen concentrations through a hurricane or atmospheric river is, by this measure, a stable system.

To quantify that stability, the team turned to a long-running treasure trove of environmental data. Five estuaries in the National Estuarine Research Reserve System — Lake Superior in Wisconsin, Chesapeake Bay’s Jug Bay in Maryland, Guana Tolomato Matanzas in Florida, Weeks Bay in Alabama, and San Francisco Bay in California — provided more than 150,000 salinity records per site and continuous 15-minute measurements of dissolved oxygen, temperature, turbidity, and water depth from synchronized sondes. Monthly grab samples supplied nutrient and chlorophyll-a concentrations. The five systems span a remarkable range of conditions, from the freshwater seiche-driven St. Louis River mouth at Lake Superior to the semi-diurnal tidal embayments of San Francisco Bay, with salinities ranging from 0.1 to 35 parts per thousand.

The researchers selected one wet and one dry year for each estuary using decades of airport precipitation records, then identified major precipitation events within those years — hurricanes Matthew and Irma, tropical storms, Nor’easters, and atmospheric rivers among them. For each event, they calculated a resistance index, a normalized value between −1 and +1 originally developed for soil ecology, which compares the shift in dissolved oxygen after a disturbance to the pre-disturbance baseline. A value of +1 signals perfect resistance; values near zero mean the storm-induced change matched the size of the baseline itself. In total, they computed resistance for dozens of events across 19 monitoring locations, then searched for patterns using linear regressions at three scales: continental, salinity-based groups, and within each individual estuary.

The headline surprise emerged immediately. San Francisco Bay, the most urbanized estuary in the study, posted the highest mean resistance at 0.66, while Weeks Bay, dominated by agricultural land, was the least resistant at 0.23. Across all five estuaries combined, resistance rose with the percentage of built area and with population density within a 10-kilometer zone around each monitoring station — a result directly opposite to the team’s original hypothesis that urbanization would erode estuarine stability. Precipitation events generally pushed dissolved oxygen downward at San Francisco Bay and Chesapeake Bay’s Jug Bay, but the shifts were small relative to baseline variability. At Lake Superior, the least urbanized system, storms actually increased dissolved oxygen significantly.

Why would concrete and population density appear to buffer an estuary against storms? The authors propose two non-exclusive explanations, both sobering. First, urban watersheds may genuinely dampen oxygen swings: increased hydrological flashiness boosts reaeration and can flush phytoplankton out of the system or suppress them through turbidity-driven light limitation. Since algal blooms generate large day-night oxygen oscillations — supersaturation by day, depletion by night — curbing phytoplankton overgrowth could keep dissolved oxygen closer to baseline. Supporting this mechanism, the study found turbidity was positively related to built area, and chlorophyll-a was negatively related to population density. Second, and more troubling, urban estuaries may simply be so chronically disturbed that a storm adds little new stress. If baseline dissolved oxygen already fluctuates wildly due to nutrient loading and wastewater inputs, an additional precipitation-driven perturbation may not register as a significant departure. In that scenario, high resistance is not health — it is the signature of a system already pushed far from its natural state.

Beyond urbanization, several physicochemical factors showed consistent associations with resistance across scales. Water column depth and turbidity were positively related to resistance, while water temperature and chlorophyll-a were negatively related, patterns that held at the continental scale, within salinity groups, and within individual estuaries. Deeper water bodies can dilute freshwater inflows, buffer temperature swings, and moderate gas exchange, though depth cuts both ways: it can also strengthen stratification and starve bottom waters of oxygen. Warm water holds less oxygen and accelerates microbial respiration, while high chlorophyll-a signals phytoplankton abundance and the volatile oxygen dynamics that come with it. The negative temperature relationship was the strongest continental-scale pattern, explaining roughly 42 percent of the variance in resistance.

Yet the deeper message of the study is that these broad generalizations dissolve as the lens zooms in. When estuaries were grouped by salinity, relationships strengthened and new predictors appeared: in high-salinity systems, resistance tracked dissolved inorganic nitrogen, the nitrogen-to-phosphorus ratio, built area, and even tree cover, with several relationships explaining more than half the variance. In low-salinity systems, cropland emerged as a significant factor. Within individual estuaries, the picture fragmented further. Resistance at Guana Tolomato Matanzas correlated with five different factors, three of which never appeared at the continental scale, while at Chesapeake Bay’s Jug Bay only water column depth mattered. Salinity was positively associated with resistance at two estuaries and negatively at a third. The direction of the depth relationship itself flipped between systems. In short, no single rulebook governs how an estuary absorbs a storm.

From these patterns the team sketched a conceptual model to guide future work. The highest resistance, they suggest, should be expected in tidal-dominated or urban-influenced estuaries that are well mixed and have short residence times, where tides rapidly homogenize storm-driven perturbations in salinity, turbidity, and nutrients — San Francisco Bay being the archetype. The lowest resistance should occur in enclosed, shallow, poorly mixed systems with long residence times and strong river or agricultural influence, such as Weeks Bay or the stratified Pellicer Creek in Florida, where storm-induced changes linger and oxygen fluctuations amplify. Most real estuaries, the authors caution, fall somewhere along this continuum, and the model is intended to generate hypotheses rather than serve as a deterministic classification.

The practical stakes are considerable. With urban populations growing and extreme precipitation intensifying across North America, coastal managers need to know which estuaries are most likely to tip into hypoxia, fish kills, and disrupted nutrient cycling after major storms. This study argues that neither a blanket urbanization penalty nor a blanket urbanization exemption will do; management strategies must pair continental-scale generalizations with local knowledge of salinity regime, stratification, nutrient speciation, and watershed land cover. It also flags dissolved nitrogen dynamics and microbial activity as priority targets for the mechanistic research needed to explain why urban estuaries appear so storm-hardy. And it delivers a cautionary lesson in measurement itself: an ecosystem can look remarkably stable precisely because it has already been degraded. Resistance, the authors remind us, is a normalized number that says nothing about whether the underlying ecological state is thriving. As storms grow stronger and cities expand, distinguishing genuine resilience from the numbness of chronic disturbance may become one of the most important tasks in coastal science.

Subject of Research: Estuarine resistance to precipitation events across urbanization gradients and spatial scales

Article Title: Physicochemical and urban land-use characteristics associated with resistance to precipitation in estuaries vary across scales

Article References: Physicochemical and urban land-use characteristics associated with resistance to precipitation in estuaries vary across scales. (n.d.). https://doi.org/10.5194/bg-23-7009-2026

Image Credits: AI Generated

DOI: 10.5194/bg-23-7009-2026

Keywords: estuaries, dissolved oxygen, urbanization, precipitation, resistance index, land use, nutrients, chlorophyll-a, salinity, water quality, Biogeosciences, coastal ecosystems