Michael S. Wetz

An 'extreme' future for estuaries? Effects of extreme climatic events on estuarine water quality and ecology.

Recent climate observations suggest that extreme climatic events (ECE; droughts, floods, tropical cyclones, heat waves) have increased in frequency and/or intensity in certain world regions, consistent with climate model projections that account for mans influence on the global climate system. A synthesis of existing literature is presented and shows that ECE affect estuarine water quality by altering: (1) the delivery and processing of nutrients and organic matter, (2) physical-chemical properties of estuaries, and (3) ecosystem structure and function. From the standpoint of estuarine scientists and resource managers, a major scientific challenge will be to project the estuarine response to ECE that will co-occur with other important environmental changes (i.e., natural climate variability, global warming, sea level rise, eutrophication), as this will affect the provisioning of important ecosystem services provided by estuaries.

Air‐water CO2 fluxes in the microtidal Neuse River Estuary, North Carolina

[1]xa0From June 2009 to July 2010, we conducted 27 continuous-flow surveys of surface water CO2 partial pressure (pCO2) along the longitudinal axis of the Neuse River Estuary (NRE), North Carolina ranging from the tidal freshwater region to the polyhaline border with the Pamlico Sound. Lateral transects were also conducted at the borders of each of three hydrologically distinct sections. The pCO2 displayed considerable spatial-temporal variability. Likewise, net air-water CO2 fluxes showed high spatial and temporal variability, with a maximum [release] of 271 mmol C m−2 d−1 during high river flow conditions in fall and minimum [uptake] of −38 mmol C m−2 d−1 during wind-driven, high primary productivity conditions in late spring. During high-flow conditions, pCO2 generally decreased from the river mouth to the Pamlico Sound, similar to patterns seen in well-mixed systems. During warm, low-flow conditions, surface water pCO2 distributions were spatially variable and dissimilar to those patterns seen in most macrotidal, well-mixed estuaries. The annual air-water CO2 efflux from the study area was 4.7 mol C m−2 yr−1, an order of magnitude less than previously estimated for temperate estuaries. The CO2 fluxes observed in the NRE highlight the contrasts between macrotidal and microtidal systems and suggest that global estuarine CO2 emissions are likely overestimated by the current classification approaches. Scaling this lower efflux by the relative surface area of macrotidal and microtidal systems would reduce the global estuarine flux by 42%.

Water quality dynamics in an urbanizing subtropical estuary(Oso Bay, Texas)

Results are presented from a study of water quality dynamics in a shallow subtropical estuary, Oso Bay, Texas, which has a watershed that has undergone extensive urbanization in recent decades. High inorganic nutrient, dissolved organic matter and chlorophyll concentrations, as well as low pH (<8), were observed in a region of Oso Bay that receives wastewater effluent. Despite being shallow (<1 m) and subjected to strong winds on a regular basis, this region also exhibited episodic hypoxia/anoxia. The low oxygen and pH conditions are likely to impose significant stress on benthic organisms and nekton in the affected area. Signatures of eutrophied water were occasionally observed at the mouth of Oso Bay, suggesting that it may be exported to adjacent Corpus Christi Bay and contribute to seasonal hypoxia development in that system as well. These results argue for wastewater nutrient input reductions in order to alleviate the symptoms of eutrophication.

Moving Forward in a Reverse Estuary: Habitat Use and Movement Patterns of Black Drum ( Pogonias cromis ) Under Distinct Hydrological Regimes

Understanding the effects of freshwater inflow on estuarine fish habitat use is critical to the sustainable management of many coastal fisheries. The Baffin Bay Complex (BBC) of south Texas is typically a reverse estuary (i.e., salinity increases upstream) that has supported many recreational and commercial fisheries. In 2012, a large proportion of black drum (Pogonias cromis) landed by fishers were emaciated, leading to concerns about the health of this estuary. In response to this event and lacking data on black drum spatial dynamics, a 2-year acoustic telemetry study was implemented to monitor individual-based movement and seasonal distribution patterns. Coupled with simultaneous water quality monitoring, the relationship between environmental variables and fish movement was assessed under reverse and “classical” estuary conditions. Acoustic monitoring data suggested that the BBC represents an important habitat for black drum; individuals exhibited site fidelity to the system and were present for much of the year. However, under reverse estuary conditions, fish summertime distribution was constrained to the interior of the BBC, where food resources are limited (based on recent benthic sampling), with little evidence of movement across the system. Out of eight environmental variables used to model fish movement using multiple linear regression, the only significant variable was salinity, which exhibited a negative relationship with movement rate. These findings suggest that prolonged periods of hypersalinity, which are detrimental to other euryhaline species due to increased osmoregulatory costs, reduce black drum distribution patterns and can limit the species’ access to benthic habitats supporting abundant prey resources.

Phytoplankton Spatial Variability in the River-Dominated Estuary, Apalachicola Bay, Florida

In shallow estuaries with strong river influence, the short residence time and pronounced gradients generate an environment for plankton that differs substantially in its dynamics from that of the open ocean, and the question arises “How is phytoplankton biomass affected?” This study assesses the small-scale spatial and temporal distribution of phytoplankton in Apalachicola Bay, a shallow bar-built estuary in the Florida Panhandle. Phytoplankton peaks were characterized to gain insights into the processes affecting spatial heterogeneity in biomass. Chlorophyll a (Chl a) distribution at 50-m spatial resolution was mapped using a flow-through sensor array, Dataflow©, operated from a boat that sampled four transects across the bay every 2xa0weeks for 16xa0months. Chl a peaks exceeding background concentrations had an average width of 1.3u2009±u20090.7xa0km delineated by an average gradient of 3.0u2009±u20096.0xa0μg Chl a L−1xa0km−1. Magnitude of E-W wind, velocity of N-S wind, tidal stage, and temperature affected peak characteristics. Phytoplankton contained in the peaks contributed 7.7u2009±u20092.7% of the total integrated biomass observed along the transects during the study period. The river plume front was frequently a location of elevated Chl a, which shifted in response to river discharge. The results demonstrate that despite the shallow water column, river flushing, and strong wind and tidal mixing, distinct patchiness develops that should be taken into consideration in ecological studies and when assessing productivity of such ecosystems.

Biogeochemistry of a River-Dominated Estuary Influenced by Drought and Storms

Increased frequency and severity of droughts, as well as growing human freshwater demands, in the Apalachicola-Chattahoochee-Flint River Basin are expected to lead to a long-term decrease in freshwater discharge to Apalachicola Bay (Florida). To date, no long-term studies have assessed how river discharge variability affects the Bay’s phytoplankton community. Here a 14-year time series was used to assess the influence of hydrologic variability on the biogeochemistry and phytoplankton biomass in Apalachicola Bay. Data were collected at 10 sites in the bay along the salinity gradient and include drought and storm periods. Riverine dissolved inorganic nitrogen and phosphate inputs were correlated to river discharge, but chlorophyll a (Chl a) was similar between periods of drought and average/above-average river discharge in most of the Bay. Results suggest that the potentially negative impact of decreased riverine nutrient input on Bay phytoplankton biomass is mitigated by the nutrient buffering capacity of the estuary. Additionally, increased light availability, longer residence time, and decreased grazing pressures may allow more Chl a biomass to accumulate during drought. In contrast to droughts, tropical cyclones and subsequent increases in river discharge increased flushing and reduced light penetration, leading to reduced Chl a in the Bay. Analysis of the time series revealed that Chl a concentrations in the Bay do not directly mirror the effect of riverine nutrient input, which is masked by multiple interacting mechanisms (i.e., nutrient loading and retention, grazing, flushing, light penetration) that need to be considered when projecting the response of Bay Chl a to changes in freshwater input.