Biology
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Item The long-term change of Chesapeake Bay hypoxia: impacts of eutrophication, nutrient management and climate change(2019) Ni, Wenfei; Li, Ming; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Eutrophication-induced coastal hypoxia can result in stressful habitat for marine living resources and cause great economic losses. Nutrient management strategies have been implemented in many coastal systems to improve water quality. However, the outcomes to mitigate hypoxia have been mixed and usually small when only modest nutrient load reduction was achieved. Meanwhile, there has been increasing recognition of climate change impacts on estuarine hypoxia, given estuaries are especially vulnerable to climate change with multiple influences from river, ocean and the atmosphere. Due to the limitation of observational studies and the lack of continuous historical data, long-term oxygen dynamics in response to the changes of external forces are still not well understood. This study utilized a numerical model to quantitatively investigate a century of change of Chesapeake Bay hypoxia in response to varying external forces in nutrient inputs and climate. With intensifying eutrophication since 1950, model results suggest an abrupt increase in volume and duration of hypoxia from 1950s-1960s to 1970s-1980s. This turning point of hypoxia might be related with Tropical Storm Agnes and consecutive wet years with relatively small summer wind speed. During 1985-2016 when the riverine nutrient inputs were modestly decreased, the simulated bottom dissolved oxygen exhibited a statistically significant declining trend of ~0.01 mgL-1yr-1 which mostly occurred in winter and spring. Warming was found to be the dominant driver of the long-term oxygen decline whereas sea level rise had a minor effect. Warming has overcome the benefit of nutrient reduction in Chesapeake Bay to diminish hypoxia over the past three decades. By the mid-21st century, the hypoxic and anoxic volumes are projected to increase by 10-30% in Chesapeake Bay if the riverine nutrient inputs are maintained at high level as in 1990s. Sea level rise and larger winter-spring runoff will generate stronger stratification and large reductions in the vertical oxygen supply to the bottom water. The future warming will lead to earlier initiation of hypoxia, accompanied by weaker summer respiration and more rapid termination of hypoxia. The findings of this study can help guide climate adaptation strategies and nutrient load abatement in Chesapeake Bay and other hypoxic estuaries.Item Thermal Physiology in a Widespread Lungless Salamander(2018) Novarro, Alexander Joseph; Bely, Alexandra E; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Understanding species responses to climate change has become a top priority for conservation biologists. Unfortunately, current models often treat species as a single entity, ignoring population-level variation. This approach may result in major oversights when managing widespread species, which generally exhibit physiological variation across their geographic range. The eastern red-backed salamander (Plethodon cinereus) is the most widely distributed Plethodon species, extending farther north than any other lungless salamander. This species’ geographic distribution raises two major questions: How does P. cinereus thrive across a wide range of temperatures, and does it possess thermal adaptations that will buffer the ecological consequences of climate change? To explore these questions, I first examined the effects of elevated temperature on metabolic hormone release rates and physiological performance (i.e., ingestion rate and mass gain) across a latitudinal population gradient. I found that physiological traits and populations differ in their thermal flexibility, and that salamanders from warmer localities are more resilient to elevated temperatures. Second, I performed a study to disentangle the environmental and evolutionary drivers of thermal limits across the geographic range of P. cinereus. I found strong support for evolutionary constraints on lower thermal limits, though there was some degree of plasticity in relation to local environmental temperatures. By contrast, upper thermal limits showed little variation across the species’ geographic range and among clades, and far exceeded survival requirements. Third, I combined laboratory experiments, field observations, and population models to explore the role of behavioral thermoregulation in shaping physiological performance in P. cinereus. I found that individuals are likely to exploit moist conditions at the cost of reduced performance, and that populations living in poor thermal quality habitats have greater thermoregulatory accuracy. Overall, my work demonstrates significant variation in thermal physiology across the geographic range and among lineages of P. cinereus and shows that thermal traits differ in their responsiveness to thermal variability. Together, these results highlight the importance of considering multiple physiological metrics and sampling large geographic areas to understand species’ abundance and distributions, and to assess species’ vulnerability to climate change.Item Response of the coastal ocean and estuaries to tropical cyclones(2018) Zhang, Fan; Li, Ming; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Landfalling tropical cyclones (TC) pose great threats to public safety. The recent decades have witnessed major advances of knowledge in TC dynamics and improvement in TC forecast models, however, occasionally inaccurate TC intensity and storm surge predictions remain a vital concern. Different representations of subgrid-scale physics by various atmospheric model parameterization schemes lead to uncertainty in predictions of TC’s intensity and associated surges. In a case study for Hurricane Arthur (2014), local closure scheme for planetary boundary layer turbulence produces lower equivalent potential temperature than non-local closure schemes, leading to under-predicted TC intensity and surge heights. On the other hand, higher-class cloud microphysics schemes over-predict TC intensity and surge heights. Without cumulus parameterization for coarse-resolution grids, both TC intensity and surge heights are grossly under-predicted due to large precipitation decreases in the storm center. To avoid widespread predictions, the ensemble mean approach is shown to be effective. Another source of TC forecast error is inaccurate sea surface temperature (SST) prediction, and accurate SST prediction necessitates a better understanding of mixing processes in the coastal ocean. Previously, the importance of TC-induced near-inertial currents (NICs) to mixing in the coastal ocean was overlooked. With high-frequency radar and autonomous glider, long-lasting NICs with amplitudes of ~0.4 m s-1 were observed on the shelf during Arthur. With an atmosphere-ocean model, we find the NICs were dominated by mode-1 vertical structure and were a major contributor to the shear spectrum. Therefore, NICs may be important in producing turbulent mixing and surface cooling during Arthur’s passage. In the future, with warmer SST, sea level rise, and possible hard shorelines in estuaries, increased storm surge hazard is expected. Using Isabel (2003) as a case study, we find storm intensification under 2100 SST raises surge heights in Chesapeake Bay by 0.1-0.4 m given increased energy input. While sea level rise in 2100 reduces surge heights by 0-0.15 m through non-linear processes, it increases total water level by 0.4-1 m. Moreover, hard shoreline further increases surge heights by up to 0.5 m in the middle and upper Chesapeake Bay by prohibiting energy flux towards wetlands.Item USING AN INDIVIDUAL BASED MODEL TO EVALUATE THE EFFECTS OF CLIMATE CHANGE ON THE REPRODUCTIVE PHENOLOGY OF EELGRASS (ZOSTERA MARINA L.) ALONG A LATITUDINAL GRADIENT(2017) Foley, Jessica Lynn; Harris, Lora A; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)I explored the effects of climate change on the reproductive biology of the clonal marine angiosperm Zostera marina L. (eelgrass) using an individual-based model. The model captures whole plant ontogeny, morphology, and ecophysiology from seed to reproductive adult to simulate the plasticity of eelgrass in response to environmental variables. Using a latitudinal gradient as a proxy for climate change, virtual seeding experiments were performed in three locations along the East coast of the United States. I simulated the impacts of increased temperatures on Z. marina’s biomass, reproductive phenology, and life history. Warmer temperatures resulted in a modeled decrease of Z. marina’s total biomass, as well as altered reproductive timing and strategy. These results have implications for long term predictions of Z. marina persistence in its traditional biogeographic range, and indicate adaptation via shifts in phenology and reproductive strategy may interact to dampen some negative consequences of increased temperatures.Item IMPACTS OF WEIGHTING SCHEMES AND TRANSFORMED ENVIRONMENTAL VARIABLES ON BIODIVERSITY MODELING WITH PRESENCE-ONLY DATA(2017) Pradhan, Kavya; Fitzpatrick, Matthew C; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Biodiversity modeling techniques at the community- and species-level can be used to address questions in ecology, management, and conservation. I addressed aspects of community-level and specie-level models using virtual and inventoried species in North and South America. Firstly, I assessed the effectiveness of two weighting schemes in reducing impacts (if any) of five sampling routines (simulating unrepresentative sampling in presence-only data) on the model performance of Generalized dissimilarity model (GDM). Unrepresentative sampling lowers model performance, but weighting species can reduce this negative impact to a certain extent. However, PO data severely impacts GDM’s ability to detect the relative contribution of environmental gradients. Secondly, I examined the potential of (GDM) transformed environmental variables in improving the performance of Maxent models (presence-only) along with the influence of range size, sample size, and species dependence type. Transformed environmental variables improved model performance, especially when used with small-ranged species and/or low sample sizes.Item Marine Community Assembly in a Dynamic Ecotone(2016) Johnston, Cora Ann; Gruner, Daniel S; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Species distributions are shifting with climate change. By altering the presence and distribution of biogenic foundation species, climate change effectively modifies habitat. Where biogenic habitats meet, a patchy ecotone landscape forms. The impacts that range shifts and habitat modification have on broader ecological communities will depend in part on how communities assemble in frontier landscapes of patchy habitat. Here, as a case study, I investigate marine fauna community formation and habitat associations along a wetland ecotone in which tropical mangroves invade temperate saltmarsh. When foundation species shift ranges, resulting changes in geographic context and local conditions will affect the contributions of dispersal limitation and species sorting to assembly. By evaluating the presence of community structure – grouping of species – in larval supply and settlers in each pure landscape and into the ecotone, I determine that ecotone marine communities are shaped by habitat-based sorting but not dispersal limitation. Where inhabitant species can access the ecotone, the attributes that inform habitat use and the scale(s) at which inhabitants distinguish between habitat types within an ecotone should determine the apparency of emerging patches along the range edge, affecting the precision with which inhabitants occupy them. I monitored marine fauna within an experimental array that isolated physical structure from broader habitat patch attributes, revealing that nested scales of habitat sensitivity should result in increasing community divergence as habitat patches expand along the range edge. Finally, habitat associations at settlement may be driven by preference or survival. I determine habitat-specific recruitment patterns of Callinectes spp. (Decapoda: Portunidae) crabs in the ecotone and use lab trials to determine that associations are driven by preference for and superior survival in vegetation with branched architecture. Together, these results demonstrate that marine fauna are sensitive to changes in structural attributes and fine-scale emergence of mangrove habitat within marshes, which do not provide equivalent habitat. This work also contributes to our understanding of community formation in a transitional landscape, illuminating the influence of patchy foundation species expansion on community-structuring ecological processes.Item CARRY-OVER EFFECTS FROM THE NON-BREEEDING SEASON INFLUENCE SPRING ARRIVAL DATES, REPRODUCTIVE SUCCESS, AND SURVIVAL IN AN ENDANGERED MIGRATORY BIRD, THE KIRTLAND'S WARBLER (SETOPHAGA KIRTLANDII)(2013) Rockwell, Sarah Mary; Inouye, David; Marra, Peter P; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)I used a unique model system to evaluate potential sources of population limitation in migratory birds that travel seasonally between temperate breeding grounds and tropical non-breeding grounds. The Kirtland's warbler (Setophaga kirtlandii) is an endangered species that is well-studied during its breeding season in northern Michigan, but how winter events may carry over to affect demographic processes remains unknown. Stable-isotope analyses of birds' tissues collected upon spring arrival revealed inconsistent yearly effects of δ13C, δ15N, and δD signatures on arrival schedules. Males departing from more mesic winter habitats (depleted δ13C), consuming more insects (enriched δ15N), and wintering further south (enriched δD) arrived earlier to temperate breeding grounds. However, these patterns were not significant across all years. Differences in δ13C and δ15N signatures between age classes were suggestive of age-related dominance relationships in winter. Winter habitat and diet differences among males within years did not seem to have strong carry-over effects, but mean spring arrival dates were delayed following drier winters, suggesting that carry-over effects in this species may be driven more by interannual variation in rainfall. There was a strong male age by rainfall interaction, with first-time breeders responding more strongly to changes in rainfall than experienced adults. Regardless of age, drier winters and delayed arrival and nest initiation were significantly associated with fewer offspring fledged in the subsequent breeding season. Analysis of our 5-year mark-recapture dataset showed that apparent survival is also positively associated with winter rainfall in the Bahamas. Kirtland's warbler survival probabilities are relatively high within the summer and winter stationary periods, but are lower during migration. Combining my survival and productivity data into a simple projection model revealed that the population growth rate of this species may become negative if Bahamas March rainfall drops more than 18% from its current mean. Climate change models predict continued drying trends in the Caribbean, which could have negative consequences on the population dynamics of the Kirtland's warbler. These results emphasize the importance of understanding the effects of climatic variation on demographic rates, and underscore the need for continued research on the ecology of migratory animals throughout the annual cycle.Item Downscaled Climate Change Forecasting and Maryland's Forests(2011) Juchs, Stephanie; McIntosh, Marla S; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Effective planning and management of forests in a changing climate requires valid and robust predictions of future climate change that are context-specific since climate changes vary by region. Climate models are often used to predict future trends in temperature and precipitation at the global level, but are most useful if downscaled to predict change at regional levels. Monthly temperature and precipitation were predicted using three downscaled regional climate models for the 1990s and the 2050s. Comparison of the 1990's predictions to weather station data from across Maryland indicated inherent model biases affecting accurate predictions, which were used to adjust the model-projected climate variables for the 2050s. The projected daily temperatures were also used to calculate projected growing degree days and frost days. The degree of climate change in Maryland projected by these regional models for the next half-century would have profound impacts on forests across Maryland.