Our Students and Their Research

Temporal and geographical trends in the sizes of members of the four dominant bivalve species (Nuculana radiata, Nuculana pernula, Macoma calcarea, and Ennucula tenuis) from in the northern Bering Sea in the St. Lawrence Island Polynya (SLIP) from 2012-2015 were studied. Clams were collected annually in July 2012-2015, and length and weight measurements were made on each individual clam collected. There was a significant decline in dominant bivalve length and weight over the 2012-2015 time period, and a significant replacement of the dominant species N. radiata by the dominant species N. pernula. In 2012, clam biomass was significantly higher in the northernmost stations (SLIP4 and SLIP4) than in the more southern stations (SLIP stations 1-3), but in 2015 clam biomass was more evenly distributed across the five stations. These changes in clam size and species composition over time may reflect population responses to ecosystem changes and food web re-structuring in the northern Bering Sea, particularly within the context of climate change. This length-weight dataset from 2012-2015 for dominant bivalves fills a gap in a larger dataset for benthic biomass which exists for the last 25 years in the SLIP.

Blog Posts:

See Sophie's posts to Fellowship Experiences, Maryland Sea Grant's blog written by and about fellows and their research.

"Exploring Climate Change in the Arctic Aboard an Icebreaker"

"Sea Ice, Sea Cucumbers, Sea Stars: My Summer Research off Alaska"

The mysid, Neomysis americana, inhabits shallow shelf waters and estuaries of the western Atlantic coast and plays a critical role in nutrient cycling and food web dynamics in these ecosystems. Despite their importance in these ecosystems, much of their spatial ecology and habitat requirements remain unknown. In Chesapeake Bay tributaries, previous research has shown that summer water temperatures can approach the lethal upper limit for N. americana. In this study, oxygen consumption rate (µg/min) of N. americana from the Patuxent River, a tributary of the Chesapeake Bay, USA, were measured experimentally as an indicator of metabolic stress under realistic temperature and salinity conditions. Mysids were incubated in a fully factorial design comprising three temperature (22, 26, 29°C) and salinity (2, 8, 16 ppt) treatments. The experimental design evaluated sex-specific responses as well as diel differences in oxygen consumption while accounting for mass-specific changes in oxygen consumption rate. Oxygen consumption rates were higher during the day for females and at low temperature (=22°C) and high salinity conditions (=16 ppt). There was no difference in oxygen consumption rate between males and females, but there was an interaction between temperature and salinity during both day and night incubations. Oxygen consumption was depressed at the lowest salinities in the 29°C treatment and in the highest salinities in the 22°C treatment (oxygen consumption rates peaked under intermediate conditions). This study indicates that the metabolic response of N. americana to temperature and salinity conditions is complex and that oxygen consumption rates can vary 3-4 fold within a realistic range of summer temperature and salinity conditions. These results also suggest that oxygen consumption rates should be only one of several indicators used to examine patterns of metabolic stress in N. americana in the wild.

The concentration of Gd-DTPA, an MRI contrasting agent, has been increasing in urban waterways over the past three decades. While Gd-DTPA is so stable in freshwater that toxic Gd3+ is unlikely to be released, little is known about the stability of this complex in marine environments. At high salinity, and thus high concentrations of Mg and Ca, Mg and Ca may compete with Gd sufficiently to release Gd3+ into the environment. In this study, the stability constants for complexes of DTPA with Ca and Mg were measured at seawater ionic strength by potentiometric titration to better understand the behavior of Gd-DTPA in marine environments. From these constants, the side-reaction coefficient for DTPA in seawater was calculated to understand the extent to which Mg and Ca compete for DTPA with trace metals and to determine conditional DTPA stability constants for seawater. This study concluded that Gd-DTPA largely dissociates in seawater, releasing toxic Gd3+.

Cable bacteria, a newly discovered sulfide-oxidizing microbe, was recently identified with genetic techniques in the rhizosphere of seagrasses. These bacteria couple the reduction of oxygen in the water with the oxidation of sulfide in sediment and are believed to improve seagrass health by reducing the amount of sulfide intrusion that occurs in below-ground plant structures. Cable bacteria were extracted from sediment taken from three sites of varying salinity and dominant seagrass species within the Chesapeake Bay watershed. These bacteria were stained with a DNA probe using fluorescent in situ hybridization to determine the density of the filamentous bacteria present. Sediment was taken from three locations within each site: from root-associated sediment, bulk surface sediment within the seagrass bed, and bulk sediment collected away from the seagrasses. The bacteria were found at all sites and were present in high density within root-associated sediment, though the differences in density were not significant between sites or sediment type. The presence of these bacteria in sediment surrounding the roots of seagrasses could identify cable bacteria as an important mechanism controlling local geochemistry of seagrass beds and maintaining the health of the plants.

In this study, two methods of sediment small core incubation, static and flow through, were compared to determine a relationship. To have results that were not site specific, sites of varying environmental conditions were used. A one-way ANOVA showed methods at the mid-bay sites to have a significant difference. While determined to be statistically insignificant, differences in shallow water sites could be seen when plotted. From the results it can be determined that there was a difference between methods although no constant trend was found.

Restoration of Crassostrea virginica, the eastern oyster, has been prioritized in large part due to the many ecosystem services that oysters provide including creating habitat and increasing water clarity due to their filtration. Yet, field observations of the ecosystem effects of C. virginica filtration on water clarity are limited. This study aimed to observe and detect C. virginica filtration over a subtidal, restored oyster reef in Harris Creek, MD. To evaluate the hypotheses that suspended solids would decrease in the direction of the tide due to oyster filtration, and that sediments would coarsen increasingly in the direction of the tide due to oyster bio-deposit resuspension and filtration, water properties, and particle size distribution were measured with an Acoustic Doppler Profiler (ADP), an optical backscatter sensor (OBS), and a Laser In-Situ Scattering Transmissometer (LISST). A clear signature of filtration was observed at maximum flood tide and maximum ebb tide: the amount of suspended solids in the water measured with the OBS and the particle volume concentrations measured with the LISST decreased over the reef in the direction of the tide. Floc particles greater than 66 um were not found to increase along the reef in the direction of the tide as was hypothesized. Instead, large patches of suspended floc particles were observed throughout the sampling period, and their source has yet to be determined. Results of this study provide evidence of the effect of C. virginica filtration in the field, and provide new evidence for patches of suspended flocs whose presence could have a significant impact on oyster filtration and associated processes.

To increase knowledge of the dynamic interactions between nitrogen fixing bacteria, submerged aquatic vegetation, marsh grass, and the surrounding environment of the Chesapeake Bay, samples of water, vegetation, and sediment were collected for analysis from eight diverse sites which varied in salinity, physical and chemical water quality and plant species. Nitrogen fixation rates were determined by performance of acetylene reduction assays using gas chromatography with a flame ionization detector and normalized to dry weight. A comparison of sample sites and microenvironments in relation to nitrogen fixation rates showed that nitrogen fixation was highest in the root zone and virtually absent from the water column during sampling dates. Anaerobic conditions increased N2 fixation rates in benthic samples. Positive correlations to fixation rates compared to sediment organic content and water column ammonia levels were also found. A persistent bloom of filamentous Lyngbya wollei dominated area N2 fixation rates in the Susquehanna Flats Vallisneria bed, causing trends divergent from other sites. Despite concerns of possible vegetation shading from floating Lyngbya mats and fixation driven eutrophication, the bed was dense and healthy with blades longer than those at any other site and encouragingly low nutrient concentrations. Overall, results point to relatively healthy ecosystems where nitrogen fixing bacteria exist in symbiosis with local vegetation.

Forage fish are important planktivorous prey species within the Chesapeake Bay for many of the larger fish, birds, and mammals that inhabit the estuarine ecosystem. However, little is known about the distribution of these fish between the rivers and creeks that feed the Bay. A maximum of five transects were performed during each of two cruises, two in the Patuxent River and three in Saint Leonard Creek, where sonar data recorded fish abundance in the water column, which were then counted and paired with GPS data to provide a dataset with high spatial resolution, allowing fish abundances to be compared in and between transects, as well as between the creek and the river. Qualitatively, the forage fish prefer the creek to the river because of the abundant source of plankton as well as the lower risk of predation from larger piscivorous fish species. However, large schools of forage fish may prefer the river due to mobility. The sea nettle Chrysaora quinquecirrha may also impact forage fish spatial habits, but this data has not been analyzed yet. Future research includes this data, as well as examining the fish abundance at a higher temporal resolution to see more conclusive trends. 

This article outlines an investigation of oyster filtration formulations, which analyzed the changes in biogeochemical model simulations that resulted from different parameterizations of oyster filtration. Through controlled sensitivity analysis on the different aspects of modeled oyster filtration, we were able to identify and quantify variations in the model response that result directly from different expressions of the filtration rate. With numerical models being widely used to investigate the Chesapeake Bay region, this analysis yields further insight into their robustness and the necessity of accurate representations of the oysters’ physical processes.

²¹⁰Pb and ⁷Be radioisotopes can be used to determine historical sedimentation rates on decadal and seasonal scales. In the context of remediated agricultural lands, such sedimentation rates can allow for the observance of changes in land use practices over time. Over the past ~100 years Trippe Creek watershed saw multiple changes in land use with the most recent (~20 years ago) being the implementation of U.S. Department of Agriculture conservational programs. This study used ²¹⁰Pb and ⁷Be analyses to examine changes in sedimentation rates in Trippe Creek. The purpose of the analysis was to determine if the implementation of conservational practices was able to lower the input of sediment into the creek. The results from the analysis were compared to²¹⁰Pb and ⁷Be results from nearby Goldsborough Creek where the watershed is still dominated by active agriculture. Sedimentation rates in Trippe Creek decreased within the past 20 years while sedimentation rates in Goldsborough Creek increased. This comparison appeared to indicate that the implementation of conservational practices did relate to the decrease in sedimentation rates. 

Continuing poor water quality and high nutrient loading promoted the establishment of the Chesapeake Bay Total Maximum Daily Load (TMDL) and the development of Watershed Implementation Plans (WIPs). Calvert County, MD is expected to allocate $1.2 billion by 2025 in restoring the watershed. Awareness of the sources of excess nitrogen is a critical factor in establishing an appropriate management practice. We sampled water from Hellen Creek, St. Leonard Creek, and Island Creek of the lower Patuxent River and filtered for nutrients. Nitrogen box models were developed for the creeks to determine their greatest input of nitrogen. It was found that during the month of July the greatest input of nitrogen to these tidal creeks is from the Patuxent River. The nitrogen loading for Hellen Creek and Island Creek from the Patuxent were 10 kg/day and 6 kg/day, respectively. These data reflect the behavior of the system in the summer month however they may not correctly reflect the behavior during other months.

Acartia tonsa are copepods prevalent in the Chesapeake Bay and serve as a major prey component in estuarine food webs as well as aquaculture. Two genetically distinct cryptic species of A. tonsa have been identified in the Chesapeake Bay. These morphologically similar lineages, F and S lineage, appear to show preference to salinity levels with the F lineage preferring low salinities and the S lineage preferring a more saline environment. Research was conducted to determine a difference in lineage preference and tolerance to stressors of hypoxic conditions at different temperatures. Results showed A. tonsa of both lineages had a high tolerance to hypoxia at a temperature of 25°C and an increased mortality rate in a 30°C environment. Further tests and are needed to determine a significant difference in F and S lineage survivorship in the 30°C hypoxic conditions. Additionally, during the study population lineage proportions were determined from collection sites at varying salinities along the Choptank River. These F and S lineage proportions reiterated the salinity preferences of the lineages but also indicated a decrease in the prevalence of F lineage A. tonsa in the Choptank River since previous records in 2003. 

This study examines the role of vegetation composition on MeHg production in the Kirkpatrick Marsh, which is located at the head of the Rhode River. The Kirkpatrick Marsh has a diverse plant community but the community tends to be distributed in patchy assemblages. This study assesses whether or not the distinct plant community assemblages influence the biogeochemical environment around the roots and specifically the quality of dissolved organic matter (DOM), which has been shown to influence mercury cycling on larger scales. In order to accomplish this, we collected 33 pore water samples from 11 plots along two transect lines, covering the four plant communities that dominate in those specific areas in the marsh. Although vegetation removal and vegetation root experiments have been conducted, pore water species-specific vegetation studies have not been conducted in the Kirkpatrick Marsh. We collected pore water samples for MeHg, T-Hg, methane, chloride, sulfate, sulfide, CDOM (Colored Dissolved Organic Matter), and DOC (Dissolved Organic Carbon) on two dates: transect 1(TR1 6/23/2016), transect 2 (TR2 6/29/2016). Our results suggested that Phragmites australis was responsible for contributing higher quality CDOM, compared to the other plant species we examined in the marsh. Additionally, no evidence was discovered to suggest that any plant species was contributing a higher quantity of DOC to the marsh. Total mercury concentrations were significantly higher in plots containing Phragmites. As a result, we suggest that measures need to be instilled to monitor the spread of the invasive species, Phragmites australis, as it has the potential to take in inorganic and elemental mercury impacting the local pore water thereby maybe a growing source of mercury to the Chesapeake Bay. While MeHg concentrations were not statistically significantly higher than other vegetation types, on the date sampled, this was driven by the site closest to the land suggesting location was somehow altering MeHg accumulation in the pore water at this site. Further study is needed as the Phragmites community is the most biogeochemicaly favorable for methylation whose expansion could also greatly influence MeHg export.

Climate data sets available currently, whether these are paleo-reconstructions, long-term weather monitoring records, or remote sensing data, contain a whelm of space-time information that needs to be analyzed under the pressure of computational and data storage requirements. This has led to a spark of interest in dynamic space-time clustering algorithms that are particularly suitable in the analysis of data streams. The trend-based clustering algorithm TRUST allows for space-time clustering in real time. However, this method requires the user to set a number of tuning parameters by hand. Here we propose a data-driven approach to automatically select the tuning parameters based on a penalized loss function. We focus on the two most important parameters of the TRUST algorithm: the current likeness of observations across the slide level and the temporal persistence within an analyzed window. We demonstrate the performance of the enhanced clustering procedure using simulated time series and illustrate its applicability using long-term records of water temperature in the Chesapeake Bay.

Throughout the last two centuries a variety of anthropogenic forces have drastically altered the Chesapeake Bay ecosystem. Because of their correlation to water quality, phytoplankton populations are useful gauges of restoration efforts and the changing biogeochemical properties of the Bay. Estuaries are dynamic environments, and traditional, in situ sampling lacks the spatiotemporal resolution necessary to effectively monitor water quality in general and phytoplankton populations in particular. In the past two decades, satellite technology has allowed for more resolved water quality measurements. However, extant ocean-observing satellites are best suited to the open ocean. In coastal and estuarine waters, high turbidity and land interference severely diminish the utility of satellite data. Hyperspectral instruments measure ocean color with much greater spectral resolution than the multispectral instruments currently deployed on ocean-observing satellites. Increased spectral resolution allows for better approximation of phytoplankton concentrations in optically complex waters. This research seeks to test optical inversion algorithms designed for the Chesapeake Bay. By pairing in situ measurements of chlorophyll with electromagnetic spectrum measurements, we determined the most accurate algorithms for use in the Chesapeake Bay. These algorithms can then be employed in future remote sensing studies to yield accurate chlorophyll measurements in fine temporal, and spatial time scales.

Mixotrophic plankton are capable of obtaining their energy through photosynthesis and phagocytosis, and have been observed to be common among marine and freshwater dinoflagellates. The role of mixotrophic dinoflagellates in the ‘microbial loop’ has received little attention.  Organisms that were only thought to introduce new carbon into the loop through photosynthesis may also consume fixed carbon by ingesting bacteria, making the ‘microbial loop’ more complex that originally conceived. The nanodinoflagellate Heterocapsa rotundata was cultured under various light and nutrient regimes to investigate the role of phototrophy and phagotrophy during winter conditions in the Chesapeake Bay. We quantified grazing rates of H. rotundata on bacteria using two feeding methods, ingestion of polycarbonate microspheres and prey removal experiments. Ingestion of fluorescent microspheres by H. rotundata revealed their ability to phagocytize particles. Using flow cytometry we calculated grazing rates of H. rotundata on bacteria under various light intensities and ammonium concentrations and found that H. rotundata increased phagotrophy at lower light intensities and ammonium was positively correlated with the grazing rates of H. rotundata. We conclude that H. rotundata uses mixotrophy as a primary source for obtaining carbon during the winter when there is limited light and low temperatures.

Blog post:

See Alison's post to Fellowship Experiences, Maryland Sea Grant's blog written by and about fellows and their research: "This Chesapeake Bay Phytoplankton Finds Multiple Ways to Snack."

Climate change escalates frequency and intensity of extreme weather events such as excessive precipitation and high wind speeds. We use Norwegian data for insurance claims and precipitation levels to identify precipitation thresholds that trigger increased number of house insurance claim and develop a statistical time series model for the claims-precipitation relationship. We evaluate a new data-driven tail comparison method to quantify future change in extreme precipitation and forecast the future change of claims’ counts in the time series model framework. The results show a location-specific change in a number of flood related insurance claims expected based on different climate change scenarios.

For this experiment, Acartia tonsa was collected from the Choptank River at the Horn Point Laboratory in Cambridge, Maryland. Five individual experiments were done on male and female A. tonsa copepods, each with 3 replicate vials. Filtered Penicillin-Streptomycin Solution antibiotic seawater was used in each experiment and treated with nitrogen gas to create oxygen concentrations ranging from 1 mg L^-1, 2 mg L^-1, 3 mg L^-1, 4 mg L^-1, and 6mg L^-1. The vials were then connected to a Pyro Science FireStingO2 Optical Oxygen Meter and placed in a 23.5ºC ± 0.5 water bath, and the oxygen concentrations in each vial was recorded for 15 hours.

The results showed that above the critical environmental oxygen partial pressure value of 9.57 kPa, respiration for both male and females were independent of the environmental oxygen partial pressure. Additionally, adult A. tonsa continued to respire and maintained their target respiration rate, as their respiration was independent of oxygen concentration. However, at lower environmental oxygen partial pressures, adult female copepods continued to respire rapidly decreasing the oxygen concentration in the experimental vials. Adult female A. tonsa was found to have higher respiration rates than males. When exposed to hypoxia at 2 mg L^-1, average female respiration rate was 0.00393 µg O₂ L^-1 µg dry wt.^-1 d^-1, while the average male respiration rate was 0.1876 µg O₂ L^-1 µg dry wt.^-1 d^-1.

Sea level rise will increase the salinity in some coastal wetlands. Evidence suggests resulting metabolic shifts could increase the carbon loss from these systems. Cove Point Marsh in Calvert County Maryland experienced a saltwater intrusion and a monitored recovery to freshwater. We used in situ oxygen data to calculate respiration, gross primary production (GPP), and net ecosystem metabolism during the summers of 2011 to 2015. We incubated water and sediment samples to determine component benthic and pelagic metabolism. GPP increased with increasing salinity via a logarithmic relationship (p<0.05) as did respiration (p<0.05).  NEM was not affected by salinity. Water column chlorophyll averaged 1.46 mg m^-2 while sediment chlorophyll was 3.5 mg m^-2. Water incubations resulted in a depth-integrated pelagic primary productivity of 0.996 g O₂ M^-2. The lack of correlation between NEM and salinity contradicted predictions that increasing salinity would increase carbon loss. Our results do not indicate that future salinity intrusions will contribute to positive feedback cycles of climate change. Cove Point decreased in salinity throughout our study, so potential regime shifts and other contributing factors merit consideration in future studies. 

The sediments of shallow coastal estuaries play important roles in storing, transforming, and removing nutrients from coastal ecosystems, and using quantitative models to accurately predict the response of sediment nutrient fluxes to environmental changes is crucial to environmental regulation. Microphytobenthos (MPB), photosynthetic microalgae inhabiting the sediment, are widespread in shallow areas and substantially alter the oxygen and nutrient dynamics of photic (illuminated) sediments, but past efforts to model ecosystem-scale sediment nutrient fluxes have largely neglected their effects. This study presents a sediment nitrogen model that computes seasonal cycles of diagenesis, nitrification, and denitrification, and includes a layer of MPB on the sediment surface that performs photosynthesis to characterize the effect of light exposure. Model outputs are validated against data from sediment core incubations at two sites in Chesapeake Bay. The model reproduces observed oxygen dynamics well; sediments exposed to light take up less oxygen during midsummer peaks in oxygen consumption than sediments in the dark, and become net sources of oxygen during spring and fall. Measured sediment nitrogen fluxes are more variable, and the model reproduces seasonal patterns less effectively, but overall both measured and modeled illuminated sediments supply less nitrogen to the water column than dark sediments. This model can be applied to water quality management to help develop recommendations for total maximum daily load of nitrogen to coastal systems.