Damian C. Brady

Nutrient enrichment and fisheries exploitation: interactive effects on estuarine living resources and their management

Both fisheries exploitation and increased nutrient loadings strongly affect fish and shellfish abundance and production in estuaries. These stressors do not act independently; instead, they jointly influence food webs, and each affects the sensitivity of species and ecosystems to the other. Nutrient enrichment and the habitat degradation it sometimes causes can affect sustainable yields of fisheries, and fisheries exploitation can affect the ability of estuarine systems to process nutrients. The total biomass of fisheries landings in estuaries and semi-enclosed seas tends to increase with nitrogen loadings in spite of hypoxia, but hypoxia and other negative effects of nutrient over-enrichment cause declines in individual species and in parts of systems most severely affected. More thoroughly integrated management of nutrients and fisheries will permit more effective management responses to systems affected by both stressors, including the application of fisheries regulations to rebuild stocks negatively affected by eutrophication. Reducing fishing mortality may lead to the recovery of depressed populations even when eutrophication contributes to population declines if actions are taken while the population retains sufficient reproductive potential. New advances in modeling, statistics, and technology promise to provide the information needed to improve the understanding and management of systems subject to both nutrient enrichment and fisheries exploitation.

Long-Term Trends of Nutrients and Sediment from the Nontidal Chesapeake Watershed: An Assessment of Progress by River and Season†

To assess historical loads of nitrogen (N), phosphorus (P), and suspended sediment (SS) from the nontidal Chesapeake Bay watershed (NTCBW), we analyzed decadal seasonal trends of flow-normalized loads at the fall-line of nine major rivers that account for >90% of NTCBW flow. Evaluations of loads by season revealed N, P, and SS load magnitudes have been highest in January-March and lowest in July-September, but the temporal trends have followed similar decadal-scale patterns in all seasons, with notable exceptions. Generally, total N (TN) load has dropped since the late 1980s, but particulate nutrients and SS have risen since the mid-1990s. The majority of these rises were from Susquehanna River and relate to diminished net trapping at the Conowingo Reservoir. Substantial rises in SS were also observed, however, in other rivers. Moreover, the summed rise in particulate P load from other rivers is of similar magnitude as from Susquehanna. Dissolved nutrient loads have dropped in the upland (Piedmont and above) rivers, but risen in two small rivers in the Coastal Plain affected by lagged groundwater input. In addition, analysis of fractional contributions revealed consistent N trends across the upland watersheds. Finally, total N:total P ratios have declined in most rivers, suggesting the potential for changes in nutrient limitation. Overall, this integrated study of historical data highlights the value of maintaining long-term monitoring at multiple watershed locations.

Characterizing the escape response of juvenile summer flounder Paralichthys dentatus to diel-cycling hypoxia.

Swimming speed, angular correlation and expected displacement were measured in juvenile summer flounder Paralichthys dentatus acclimated to either oxygen saturation (c. 7.8 mg O(2) l(-1); saturation-acclimated fish) or diel-cycling hypoxia (cycling between 11.0 and 2.0 mg O(2) l(-1)) for 10 days and subsequently exposed to more severe diel-cycling hypoxia (cycling between 7.0 and 0.4 mg O(2) l(-1)). Saturation-acclimated P. dentatus exhibited an active response to declining dissolved oxygen (DO) by increasing swimming speed, angular correlation and expected displacement to peak levels at 1.4 mg O(2) l(-1) that were 3.5, 5.5 and 4.2 fold, respectively, greater than those at DO saturation. Diel-cycling hypoxia-acclimated P. dentatus also exhibited an active response to declining DO, although it was relatively less pronounced. Diel-cycling hypoxia-acclimated P. dentatus swimming speed, however, still doubled as DO decreased from 7.0 to 2.8 mg O(2) l(-1). Diel-cycling hypoxia-acclimated P. dentatus did not recover as well from low DO exposure as did saturation-acclimated fish. This was reflected in their relatively more random swimming (low angular correlation between successive moves) and poor maintenance of rank order between individuals during the recovery phase. Even saturation-acclimated P. dentatus did not resume swimming at speeds observed at saturation until DO was 4.2 mg O(2) l(-1). Paralichthys dentatus were very sensitive to decreasing DO, even at DO levels that were not lethal or growth limiting. This sensitivity and their poor recovery may preclude juvenile P. dentatus from using highly productive nursery habitats affected by diel-cycling hypoxia.

Nutrient- and Climate-Induced Shifts in the Phenology of Linked Biogeochemical Cycles in a Temperate Estuary

The response of estuarine ecosystems to long-term changes in external forcing is strongly mediated by interactions between the biogeochemical cycling of carbon, oxygen, and inorganic nutrients. Although long-term changes in estuaries are often assessed at the annual scale, phytoplankton biomass, dissolved oxygen concentrations, and biogeochemical rate processes have strong seasonal cycles at temperate latitudes. Thus, changes in the seasonal timing, or phenology, of these key processes can reveal important features of long-term change and help clarify the nature of coupling between carbon, oxygen, and nutrient cycles. Changes in the phenology of estuarine processes may be difficult to assess, however, because many organisms are mobile and migratory, key primary and secondary producers have relatively rapid physiological turnover rates, sampling in time and space is often limited, and physical processes may dominate variability. To overcome these challenges, we have analyzed a 32-year record (1985-2016) of relatively frequent and consistent measurements of chlorophyll-a, dissolved oxygen, nitrogen, and physical drivers to understand long-term change in Chesapeake Bay. Using a suite of metrics that directly test for altered phenology, we quantified changes in the seasonal timing of key biogeochemical events, which allowed us to illustrate spatially- and seasonally-dependent shifts in the magnitude of linked biogeochemical parameters. Specifically, we found that a modest reduction in nitrate input was linked to a suppression of spring phytoplankton biomass in seaward Bay regions. This was, in turn, associated with an earlier breakup in hypoxia and decline in late-summer NH4+ accumulation in seaward waters. In contrast, we observed an increase in winter phytoplankton biomass in landward regions, which was associated with elevated early summer hypoxic volumes and NH4+ accumulation. Seasonal shifts in oxygen depletion and NH4+ accumulation are consistent with reduced nitrogen inputs, spatial patterns of chlorophyll-a, and increases in temperature. In addition, these increases have likely elevated rates of organic matter degradation, thus “speeding-up” the typical seasonal cycle. The causes for the recent landward shift in phytoplankton biomass and NH4+ accumulation are less clear; however, these altered patterns are analyzed here and discussed in terms of numerous physical, climatic, and biological changes in the estuary.

Model-assisted measurements of suspension-feeding flow velocities

ABSTRACT Benthic marine suspension feeders provide an important link between benthic and pelagic ecosystems. The strength of this link is determined by suspension-feeding rates. Many studies have measured suspension-feeding rates using indirect clearance-rate methods, which are based on the depletion of suspended particles. Direct methods that measure the flow of water itself are less common, but they can be more broadly applied because, unlike indirect methods, direct methods are not affected by properties of the cleared particles. We present pumping rates for three species of suspension feeders, the clams Mya arenaria and Mercenaria mercenaria and the tunicate Ciona intestinalis, measured using a direct method based on particle image velocimetry (PIV). Past uses of PIV in suspension-feeding studies have been limited by strong laser reflections that interfere with velocity measurements proximate to the siphon. We used a new approach based on fitting PIV-based velocity profile measurements to theoretical profiles from computational fluid dynamic (CFD) models, which allowed us to calculate inhalant siphon Reynolds numbers (Re). We used these inhalant Re and measurements of siphon diameters to calculate exhalant Re, pumping rates, and mean inlet and outlet velocities. For the three species studied, inhalant Re ranged from 8 to 520, and exhalant Re ranged from 15 to 1073. Volumetric pumping rates ranged from 1.7 to 7.4 l h−1 for M. arenaria, 0.3 to 3.6 l h−1 for M. mercenaria and 0.07 to 0.97 l h−1 for C. intestinalis. We also used CFD models based on measured pumping rates to calculate capture regions, which reveal the spatial extent of pumped water. Combining PIV data with CFD models may be a valuable approach for future suspension-feeding studies. Summary: Mya arenaria, Mercenaria mercenaria and Ciona intestinalis exhibit a wide range of suspension feeding rates as demonstrated by a combined experimental and numerical approach to quantifying fluid flows.

Modeling Physical and Biogeochemical Controls on Dissolved Oxygen in Chesapeake Bay: Lessons Learned from Simple and Complex Approaches

We compared multiple modeling approaches in Chesapeake Bay to understand the processes controlling dissolved oxygen (O2) cycling and compare the advantages and disadvantages of the different models. Three numerical models were compared, including: (1) a 23-compartment biogeochemical model coupled to a regional scale, salt- and water-balance box model, (2) a simplified, four-term model formulation of O2 uptake and consumption coupled to a 3D-hydrodynamic model, and (3) a 23-compartment biogeochemical model coupled to a 3D-hydrodynamic model. All three models reproduced reasonable spatial and temporal patterns of dissolved O2, leading us to conclude that the model scale and approach one chooses to apply depends on the scientific questions motivating the study. From this analysis, we conclude the following: (1) Models of varying spatial and temporal scales and process resolution have a role in the scientific process. (2) There is still much room for improvement in our ability to simulate dissolved O2 dynamics in coastal ecosystems. (3) An ever-increasing diversity of models, three of which are presented here, will vastly improve our ability to discern physical versus biogeochemical controls on O2 and hypoxia in coastal ecosystems.

Oyster Aquaculture Site Selection Using Landsat 8-Derived Sea Surface Temperature, Turbidity, and Chlorophyll a

Remote sensing data is useful for selection of aquaculture sites because it can provide water-quality products mapped over large regions at low cost to users. However, the spatial resolution of most ocean color satellites is too coarse to provide usable data within many estuaries. The Landsat 8 satellite, launched February 11, 2013, has both the spatial resolution and the necessary signal to noise ratio to provide temperature, as well as ocean color derived products along complex coastlines. The state of Maine (USA) has an abundance of estuarine indentations (~3,500 miles of tidal shoreline within 220 miles of coast), and an expanding aquaculture industry, which makes it a prime case-study for using Landsat 8 data to provide products suitable for aquaculture site selection. We collected the Landsat 8 scenes over coastal Maine, flagged clouds, atmospherically corrected the top-of-the-atmosphere radiances, and derived time varying fields (repeat time of Landsat 8 is 16 days) of temperature (100 m resolution), turbidity (30 m resolution), and chlorophyll a (30 m resolution). We validated the remote-sensing-based products at several in situ locations along the Maine coast where monitoring buoys and programs are in place. Initial analysis of the validated fields revealed promising new areas for oyster aquaculture. The approach used is applicable to other coastal regions and the data collected to date show potential for other applications in marine coastal environments, including water quality monitoring and ecosystem management.

Can TMDL Models Reproduce the Nutrient Loading-Hypoxia Relationship?

Anthropogenic nutrient enrichment of estuaries is a problem dramatically transforming coastal ecosystems worldwide. Despite significant public and private sector resources dedicated to curbing point and non-point sources of nutrient loading, many of the symptoms of eutrophication, such as low bottom water dissolved oxygen (DO), have not abated. Recently, studies have suggested that many eutrophied estuaries have exhibited unexpected responses to nutrient reduction: hypoxic volume has continued to increase while nutrient loading has plateaued or decreased. The objective of this project was to construct a long-term time series of nutrient loading for the Chesapeake Bay (1950-present) from nutrient loading observations (i.e., Susquehanna, Potomac, and Patuxent), as well as proxies for other non-tidal rivers, long-term records of point sources, and proxies for changes in atmospheric loading. This loading reconstruction was then used to force the 4,000 segment Chesapeake Bay Program Environmental Modeling Package (CBEMP). Hydrodynamic simulations were limited to three freshwater states (i.e., normal, wet, and dry). Results indicate that while the model captures relative inter-annual variability in hypoxic volume in late July, late June/early July hypoxic volume may be more difficult to capture due to the inability of these models to accurately simulate stratification. Several improvements were also suggested for the sediment flux model of the CBEMP that are already being incorporated into management efforts in the Bay. Long-term trends in nutrient loading have also been analyzed and indicate a general increase in particulate loading (Particulate Phosphorus – PP, Particulate Nitrogen – PN, and Suspended Sediment – SS) while dissolved constituents have been on the decline. This title belongs to WERF Research Report Series ISBN: 9781780406497 (eBook)