Showing papers in "Estuaries and Coasts in 2019"

Hard Structures for Coastal Protection, Towards Greener Designs

Abstract: Over recent years, many coastal engineering projects have employed the use of soft solutions as these are generally less environmentally damaging than hard solutions. However, in some cases, local conditions hinder the use of soft solutions, meaning that hard solutions have to be adopted or, sometimes, a combination of hard and soft measures is seen as optimal. This research reviews the use of hard coastal structures on the foreshore (groynes, breakwaters and jetties) and onshore (seawalls and dikes). The purpose, functioning and local conditions for which these structures are most suitable are outlined. A description is provided on the negative effects that these structures may have on morphological, hydrodynamic and ecological conditions. To reduce or mitigate these negative impacts, or to create new ecosystem services, the following nature-based adaptations are proposed and discussed: (1) applying soft solutions complementary to hard solutions, (2) mitigating morphological and hydrodynamic changes and (3) ecologically enhancing hard coastal structures. The selection and also the success of these potential adaptations are highly dependent on local conditions, such as hydrodynamic forcing, spatial requirements and socioeconomic factors. The overview provided in this paper aims to offer an interdisciplinary understanding, by giving general guidance on which type of solution is suitable for given characteristics, taking into consideration all aspects that are key for environmentally sensitive coastal designs. Overall, this study aims to provide guidance at the interdisciplinary design stage of nature-based coastal defence structures.

Evaluating the Relationship Among Wetland Vertical Development, Elevation Capital, Sea-Level Rise, and Tidal Marsh Sustainability

Abstract: Accelerating sea-level rise and human impacts to the coast (e.g., altered sediment supply and hydrology, nutrient loading) influence the accumulation of sediment and organic matter, and thereby impact the ability of coastal tidal wetlands to maintain an elevation consistently within the vegetation growth range. Critical components of marsh sustainability are the marsh elevation within the vegetation growth range (elevation capital) and the rates of marsh surface elevation change and relative sea-level rise. The relationship among these factors and their combined influence on marsh integrity were evaluated by comparing trends in surface elevation change on five salt marsh sites located on three marsh islands in Jamaica Bay, NY, USA. All marsh sites were located in a similar physical setting (i.e., tidal range, sea-level rise rate, sediment supply). The structural integrity of the marshes ranged from densely vegetated (high integrity) to severely deteriorated (low integrity) with elevation capital ranging from high to low, respectively, and included a deteriorating marsh site that was partially restored. Two marshes with high elevation capital maintained their relative position high within the tidal range through a combination of surface sediment deposition and shallow subsurface expansion, and kept pace with local sea-level rise. A marsh with moderate elevation capital showed signs of flooding stress and was deteriorating, but managed to keep pace with local sea-level rise. The deteriorated marsh gained no elevation over the 14-year study and was located too low within the tidal range to support continuous coverage of salt marsh vegetation. Elevation gain in the restored marsh initially lagged behind sea-level rise for 8 years, but the elevation trend recovered and kept pace with sea-level rise for the last 5 years. A conceptual model is presented that describes the relationship among elevation capital, and rates of marsh elevation gain and sea-level rise. Note that a search for factors influencing wetland loss should focus on process changes to marsh vertical development (e.g., sediment supply, vegetation growth) and climate change effects (e.g., sea-level and temperature rise) that can cause elevation gain to lag behind sea-level rise, and these occur prior to the onset of marsh deterioration.

Impacts of Urbanization and Development on Estuarine Ecosystems and Water Quality

Abstract: Urbanization and human-led development have increased more rapidly along shorelines and in coastal watersheds than inland regions over the past century. The result of major land use changes for both urban tracts and agriculture to serve the urban areas, as well as infrastructure development is increased runoff carrying sediments, nutrients, pollutants, pharmaceuticals, and toxins downstream to estuarine systems. The increased runoff levels are only the tip of the iceberg, with human development resulting in increased fecal bacteria from urbanization and excess nutrients from agriculture leading to harmful algal blooms. Estuaries act as a natural filter between land and sea, but have been overloaded by the influx of sediments and pollutants in recent decades. As a result, there have been a variety of impacts to estuarine ecosystems and water quality including increased sediment load, eutrophication, harmful algal blooms, fecal bacteria, as well as shellfish and fisheries declines. In some estuarine systems, the reduction in light penetration to the benthos has led to the loss of seagrasses. In others, seasonal hypoxia is a visible symptom of prolonged eutrophication. There is a need to augment long-term monitoring techniques with new technologies and data processing methods to better understand the current state of estuaries and work towards mitigating human impacts on estuarine ecosystems and water quality.

Wave Attenuation by Oyster Reefs in Shallow Coastal Bays

Abstract: Oyster reef restoration in shallow estuarine environments has been thought to have the potential to provide shoreline protection as well as oyster habitat. This study was designed to address the question of how effective oyster reefs are at attenuating wave energy in shallow coastal bays. Measurements were made of waves on both sides of four restored intertidal oyster reefs and at a control site with no reef; mean water depths ranged from 0.9 to 1.3 m. The reefs differed in composition and position relative to the shoreline, but all had reef crest elevations between 0.3 and 0.5 m below mean sea level. Differences in wave heights between the exposed/sheltered sides and upwind/downwind sides of the reefs were used to quantify the effects of the reefs on waves under varying tidal and wind conditions. All four reefs were able to reduce wave heights by an average of 30–50% for water depths of 0.5–1.0 m (bracketing the heights of reef crests) and 0–20% for water depths of 1.0–1.5 m (reef crests > 0.25 m below the water surface). For water depths greater than 1.5 m, there was < 10% change in wave heights. In contrast, there was no average decrease in wave height from the more seaward (exposed) to the more landward wave gauge at the control site regardless of water depth. Based on our results, we conclude that fringing oyster reefs can reduce the wave energy reaching the shoreline of marshes with edge elevations close to mean sea level. However, reefs like those in our study have little effect on waves during deeper water conditions, which allow for the largest waves, and are therefore less likely to offer protection to marshes characterized by high edge scarps and marsh surface elevations well above mean sea level.

Marshes Are the New Beaches: Integrating Sediment Transport into Restoration Planning

Abstract: Recent coastal storms and associated recovery efforts have led to increased investment in nature-based coastal protection, including restoration of salt marshes and construction of living shorelines. In particular, many of these efforts focus on increasing vertical elevation through sediment nourishment, where sediment is removed from the tidal channel and placed on the marsh plain, or preventing lateral erosion through shoreline protection. In the USA alone, millions of dollars have been allocated or spent on these coastal protection solutions over the last few decades because of their perceived sustainability and ecologically positive co-benefits including habitat provision and carbon sequestration. These projects would benefit from integration of sediment transport pathways, budgets, and metrics during planning and modeling of restoration outcomes, in order to evaluate sustainability before investment. This is analogous to the decades of experience with coastal management and engineering on the open coast. Salt marshes are geomorphic features that rely partially on external sediment supply to maintain their network of tidal channels, intertidal flats, and marsh plain. Removing sediment from one component of the overall system to nourish another component may be counterproductive, given that the net sediment budget is unchanged. For example, dredging a tidal channel beyond its equilibrium condition will cause it to fill with sediment from the tidal flat or elsewhere in the system. This may cause slumping of the marsh edge, or over-deepening of other sections of the channel to compensate. Similarly, shoreline protection that prevents edge erosion hampers the marsh plain’s ability to accrete on the levee and naturally transgress landward or it starves other components of the system of regularly supplied sediment. A limited vertical or lateral-only perspective, instead of a three-dimensional perspective, during project planning and evaluation may lead to suboptimal decision-making regarding restoration priorities, approaches, and outcomes. I contend that before significant investments are made in marsh restoration through sediment nourishment or shoreline protection, sediment transport measurements and models that consider sediment dynamics should be integrated into the early phases of restoration planning. This will help identify where and under what conditions marsh restoration will most likely be successful and economically justified. Triaging and prioritizing is then possible, which is a sustainable approach for restoration, given the persistent vulnerability of marshes to sea-level rise, storms, and sediment deficits.

Artificial Lighting at Night in Estuaries—Implications from Individuals to Ecosystems

Abstract: Artificial lighting at night (ALAN) produced by urban, industrial, and roadway lighting, as well as other sources, has dramatically increased in recent decades, especially in coastal environments that support dense human populations. Artificial “lightscapes” are characterized by distinct spatial, temporal, and spectral patterns that can alter natural patterns of light and dark with consequences across levels of biological organization. At the individual level, ALAN can elicit a suite of physiological and behavioral responses associated with light-mediated processes such as diel activity patterns and predator-prey interactions. ALAN has also been shown to modify community composition and trophic structure, with implications for ecosystem-level processes including primary productivity, nutrient cycling, and the energetic linkages between aquatic and terrestrial systems. Here, we review the state of the science relative to the impacts of ALAN on estuaries, which is an important step in assessing the long-term sustainability of coastal regions. We first consider how multiple properties of ALAN (e.g., intensity and spectral content) influence the interaction between physiology and behavior of individual estuarine biota (drawing from studies on invertebrates, fishes, and birds). Second, we link individual- to community- and ecosystem-level responses, with a focus on the impacts of ALAN on food webs and implications for estuarine ecosystem functions. Coastal aquatic communities and ecosystems have been identified as a key priority for ALAN research, and a cohesive research framework will be critical for understanding and mitigating ecological consequences.

Microclimate Influences Mangrove Freeze Damage: Implications for Range Expansion in Response to Changing Macroclimate

Abstract: In response to warming winter air temperatures, freeze-sensitive mangrove forests are expected to expand at the expense of freeze-tolerant salt marshes. To better anticipate and prepare for mangrove range expansion, there is a need to advance understanding of the modulating role of microclimate. Here, we synthesized hypotheses regarding the effects of microclimatic variation on temperature gradients and mangrove freeze damage. Temperature data from the literature and from temperature loggers were used to quantify ecologically relevant temperature gradients. Then, literature-derived mangrove freeze damage data were used to quantify the ecological effects of these temperature gradients. Six microclimatic factors are described that produce air temperature gradients that modulate mangrove responses to winter temperature extremes: (1) distance from the ocean; (2) distance from wind buffers; (3) mangrove canopy cover; (4) height above the soil surface; (5) local slope concavity; and (6) tidal inundation. Variation in these factors produces local temperature differences that range from 2 to 14 °C, with concomitant effects on horizontal and vertical patterns of biological damage from freezing. Collectively, our results elucidate the influence of microclimate on spatial patterns of biological damage and mortality due to winter temperature extremes. As mangrove ranges expand in response to climate change, we anticipate that microclimatic variation will produce adverse environments where mangrove expansion is prohibited as well as expansion hot spots where mangroves are protected. Subsequent expansion into newly available habitat will occur from protection zones, and microclimatic gradients may even produce positive feedback cycles that ultimately accelerate the rate of range expansion in response to warming.

Spatial Distribution and Physical Controls of the Spring Algal Blooming Off the Changjiang River Estuary

Abstract: The Changjiang River discharges huge amounts of nutrients, which cause frequent harmful algal blooms off its estuary. By analyzing historical and in situ observational data, this study aimed to determine the distribution and physical controls of these blooms. The results indicated that phytoplankton bloomed within the 30–50 m isobaths south of the Changjiang River mouth, with several high-value centers. South of the river mouth, the river plume front propagated along the coast as bottom-trapped river plume, which reached the bottom along the 25–30 m isobaths. The algae bloomed between the locations of the bottom and surface fronts. The surface front determined the seaward limit of the nutrient-rich river-influenced water and set the outer boundary of algal blooming; the latter separated the stratified (hence less turbid) offshore surface water from the well-mixed thus turbid nearshore surface water, forming the shoreward limit of the algal blooming due to high turbidity. The surface plume front was relatively unstable compared with the bottom. The surface plume front relaxes around the bathymetric irregularities, which enlarges the distance between the plume’s surface and bottom fronts, providing a larger area favorable to algal blooming. Consequently, several high chlorophyll-a centers developed around the bathymetric irregularities. East and northeast of the river mouth, the river plume floated above ambient seawater as a surface-trapped river plume. A subsurface chlorophyll-a maximum was observed above the pycnocline in the far-field plume area, which descended gradually to ~ 10 m in the offshore direction. The associated dynamical mechanism was also discussed.

Experimental Saltwater Intrusion Drives Rapid Soil Elevation and Carbon Loss in Freshwater and Brackish Everglades Marshes

Abstract: Increasing rates of sea-level rise (SLR) threaten to submerge coastal wetlands unless they increase soil elevation at similar pace, often by storing soil organic carbon (OC). Coastal wetlands face increasing salinity, marine-derived nutrients, and inundation depths from increasing rates of SLR. To quantify the effects of SLR on soil OC stocks and fluxes and elevation change, we conducted two mesocosm experiments using the foundation species sawgrass (Cladium jamaicense) and organic soils from freshwater and brackish Florida Everglades marshes for 1 year. In freshwater mesocosms, we compared ambient and elevated salinity (fresh, 9 ppt) and phosphorus (ambient, + 1 g P m−2 year−1) treatments with a 2 × 2 factorial design. Salinity addition reduced root biomass (48%), driving 2.8 ± 0.3 cm year−1 of elevation loss, while soil elevation was maintained in freshwater conditions. Added P increased root productivity (134%) but also increased breakdown rates (k) of roots (31%) and leaves (42%) with no effect on root biomass or soil elevation. In brackish mesocosms, we compared ambient and elevated salinity (10, 19 ppt) and inundated and exposed conditions (water level 5-cm below and 4-cm above soil). Elevated salinity decreased root productivity (70%) and root biomass (37%) and increased k in litter (33%) and surface roots (11%), whereas inundation decreased subsurface root k (10%). All brackish marshes lost elevation at similar rates (0.6 ± 0.2 cm year−1). In conclusion, saltwater intrusion in freshwater and brackish wetlands may reduce net OC storage and increase vulnerability to SLR despite inundation or marine P supplies.

Decadal Monitoring in Bermuda Shows a Widespread Loss of Seagrasses Attributable to Overgrazing by the Green Sea Turtle Chelonia mydas

Abstract: The condition of seagrass habitat and the marine environment on the Bermuda Platform, a mid-oceanic shallow water habitat in the northwest Atlantic, has been monitored since 2006. The overall oceanic climate of the Platform is subtropical; the Platform supports communities of tropical marine seagrasses, including Thalassia testudinum, Syringodium filiforme, Halodule sp., and Halophila decipiens. At the beginning of the study, the general condition of seagrass beds at 17 permanent offshore and nearshore sites indicated that 14 were healthy, complex, and thriving communities, and three represented offshore beds, which had declined precipitously prior to the initiation of study. Over the period of the study, seagrass beds declined at all 17 sites; three beds disappeared, and there was no recovery at the sites known to have declined prior to 2006. Over the same period, there was no apparent negative change in the water quality overlying the seagrass beds. Assessments of elemental content, stable isotopic composition, and leaf morphology indicated that grazing by the green turtle (Chelonia mydas) is driving the decline of the seagrasses of Bermuda. Given the feeding behavior of these turtles on the Bermuda Platform, human intervention may be required to mitigate the decline of seagrass in Bermuda.

The incorporation of biophysical and social components in coastal management

Abstract: Change is inherent in coastal systems, which are amongst the most dynamic ones on Earth. Increasing anthropogenic pressure on coastal zones interferes with natural coastal dynamics and can cause ecosystem imbalances that render the zones less stable. Furthermore, human occupation of coastal zones often requires an uncharacteristic degree of stability for these inherently dynamic coastal systems. Coastal management teams face multifaceted challenges in protecting, rehabilitating and conserving coastal systems. Diverse monitoring schemes and modelling tools have been developed to address these challenges. In this article, we explore various perspectives: the integration of biophysical, ecological and social components; the uncertainties of diverse data sources; and the development of flexible coastal interventions. We propose general criteria and guidance for an Ecosystem-based Management (EbM) to coastal management, which aims primarily at adaptation to global change and uncertainties, and to managing and integrating social aspects and biophysical components based on the flows of energy and matter.

Patterns and Trends in Secchi Disk Depth over Three Decades in the Chesapeake Bay Estuarine Complex

Abstract: Water clarity is an important ecosystem indicator of eutrophication in Chesapeake Bay and other coastal and estuarine systems across the globe. Although a variety of measures are available to quantify light availability in water, Secchi disk depths have been the most consistent and frequent measure employed in water monitoring programs. Because light availability is influenced by multiple variables, such as phytoplankton biomass, non-living suspended particles, and colored dissolved organic matter (CDOM), understanding the factors driving long-term variability and trends in water clarity is critical for targeting watershed management actions related to eutrophication. Thus, we conducted a comprehensive statistical analysis of spatial and temporal variations in Secchi disk depth and the key internal and external variables that influence its variability in Chesapeake Bay and its tidal tributaries over the past 30 years. Our results indicate that although watershed nutrient, sediment, and freshwater inputs did not correlate with Secchi depth on a monthly timescale outside of low-salinity regions near river outflows, water-column variables that represent the consequences of those inputs (CDOM, chlorophyll-a, and total suspended solids [TSS]) were strongly associated with Secchi depth variability. The inconsistency of these two findings may be explained by controls on chlorophyll-a and TSS that are not directly related to watershed input, such as grazing and resuspension, and by lags of several months between watershed inputs and the associated water-column concentrations. While salinity (a proxy for CDOM) was a dominant spatial covariate with Secchi depth bay-wide, TSS concentrations were strongly associated with temporal changes in Secchi depths in low-salinity regions and indicators of phytoplankton biomass were more important in mesohaline and polyhaline regions. These findings related to spatially dependent controls on Secchi depth enhance our understanding of long-term changes in estuarine light availability and suggest a region-specific response of Secchi depth to variables (TSS and chlorophyll-a) targeted by watershed restoration actions designed to limit nutrient and sediment inputs to Chesapeake Bay.

Nutrient and Trace Metal Fluxes into Estuarine Sediments Linked to Historical and Expanding Agricultural Activity (Hearnes Lake, Australia)

Abstract: Agriculture activities in coastal catchments often contribute pollutant runoff such as nitrogen (N), phosphorus (P), and trace metals to estuarine environments. Here, we determine the historical impacts from agricultural expansion using sediment samples from an estuary and a coastal wetland downstream from intensive historical banana, and now, blueberry plantations (Hearnes Lake estuary, NSW, Australia). The three 210Pb dated sediment cores and surface sediment samples analyzed in this study revealed that trace metals (As, Cd, Cu, Pb, Zn, Cr) and N fluxes recently increased as a result of increasing sedimentation rates. Several moderate localized enrichments were observed in recent sediment layers. Most importantly, we found a clear link between the sediment P profile and recent blueberry cultivation. P enrichment increased by 9-fold and sediment fluxes by over 40-fold (up to 12.6 mg m−2 year−1) during the expansion of blueberry cultivation within the catchment since 2002. The trapping of sediments enriched in P by the wetlands provided a good chronology of agricultural practices in the region. With a global and regional increase in estuarine eutrophication, this study further demonstrates the importance of agricultural sources and the role coastal wetlands play in sequestering P from runoff.

Climatic Controls on the Distribution of Foundation Plant Species in Coastal Wetlands of the Conterminous United States: Knowledge Gaps and Emerging Research Needs

Abstract: Foundation plant species play a critical role in coastal wetlands, often modifying abiotic conditions that are too stressful for most organisms and providing the primary habitat features that support entire ecological communities. Here, we consider the influence of climatic drivers on the distribution of foundation plant species within coastal wetlands of the conterminous USA. Using region-level syntheses, we identified 24 dominant foundation plant species within 12 biogeographic regions, and we categorized species and biogeographic regions into four groups: graminoids, mangroves, succulents, and unvegetated. Literature searches were used to characterize the level of research directed at each of the 24 species. Most coastal wetlands research has been focused on a subset of foundation species, with about 45% of publications directed at just one grass species—Spartina alterniflora. An additional 14 and 8% have been directed, respectively, at two mangrove species—Rhizophora mangle and Avicennia germinans. At the national scale, winter temperature extremes govern the distribution of mangrove forests relative to salt marsh graminoids, and arid conditions can produce hypersaline conditions that increase the dominance of succulent plants, algal mats, and unvegetated tidal flats (i.e., salt flats, salt pans) relative to graminoid and mangrove plants. Collectively, our analyses illustrate the diversity of foundation plant species in the conterminous USA and begin to elucidate the influence of climatic drivers on their distribution. However, our results also highlight critical knowledge gaps and identify emerging research needs for assessing climate change impacts. Given the importance of plant-mediated processes in coastal wetland ecosystems, there is a pressing need in many biogeographic regions for additional species- and functional group-specific research that can be used to better anticipate coastal wetland responses to rising sea levels and changing temperature and precipitation regimes.

Does the Functional Richness of Plants Reduce Wave Erosion on Embryo Coastal Dunes

Abstract: Coastal erosion is a natural process, whose intensity and occurrence have increased due to natural and anthropogenic factors. To protect the coasts, the use of hard infrastructure is a widespread practice that can be effective, mostly at a local scale. However, recent evidence also shows that downstream erosion can be accelerated in adjacent zones. Because of this, natural barriers such as coastal dunes and their plant cover have gained attention, but there is a general lack of information about the role that different species (and combinations of species) play in coastal protection. The aim of this study was to explore if the functional richness of plant species helps reduce wave erosion on embryo coastal dunes. In a wave flume, we set up a 1:1 scale artificial dune covered with different combinations of plant species (Ipomoeae pes-caprae, Sesuvium portulacastrum, and Sporobolus virginicus) and exposed it to simulated “storm waves”. We found that erosion was reduced in dunes covered by plants, but such protection was species-specific and the effectiveness of protection varied over time. Ipomoea was the most effective specie for protection. Differences between species and combinations of species were associated with their physical attributes such as growth form and plant architecture. Although we found that there are species that offer little or no protection from hydrodynamic forces, they may still be important for coastal protection through their ability to build embryo dunes through eolian processes. Indeed, nature-based coastal protection is likely to be an effective alternative to engineered solutions at many sites, but the protection provided is species-specific.

Reconstructing Historical Marine Populations Reveals Major Decline of a Kelp Forest Ecosystem in Australia

Abstract: Managing changing ecosystems requires an understanding both of how the system is currently performing and of how current performance relates to long-term, often variable, natural dynamics. However, making such assessments usually relies on having long-term ecological datasets, leaving managers often reliant on assumptions because such information is relatively rare. This is particularly an issue in kelp forests, as these ecosystems are difficult to survey, and many monitoring programs are relatively recent (last 10–20 years). A historical time series was constructed of kelp (Ecklonia radiata) and sea urchin (Heliocidaris erythrogramma) populations using various field-collected datasets, beginning in the 1960s, for the large shallow embayment of Port Phillip Bay, Australia. Additionally, aerial photographs were sourced to calculate the extent of these algal beds, going back to the 1930s. Both surveys and aerial images confirm that between the 1930s and the 1980s, kelp once occurred as dense (21–58% cover) beds over large sections of reef at all surveyed sites. However, by the early 2000s, kelp cover had declined by between 59 and 98%. From 2005 to 2012, sea urchins became 250–420% more abundant and were observed to be directly consuming large areas of macroalgae, creating the so-called sea urchin barrens. Analysis of reef algal cover between the 1930s and 2014 indicates that increases in temperature and declines in rainfall—which, in Port Phillip Bay, influences salinity, nutrient inputs and algal productivity—are correlated with the declines in kelp abundance. While it is difficult to tease apart all possible drivers, the years 1997–2009 coincided with a 1 °C increase in average air temperature and the longest drought period (137 mm less rainfall than the annual long-term average) in this region in recent record.

The Relative Role of Mangroves on Wave Erosion Mitigation and Sediment Properties

Abstract: Fringe mangroves face waves daily and are thought to protect against erosion in low wave energy sites and undergo erosion if exposed to high wave energy. We aimed to understand the effects of fringe mangroves on erosion and sediment dynamics and of wave exposure on seedling density at three sites of increasing wave energy. Sediment properties (mean grain size, sorting, and bulk density) were assessed within each site in unvegetated and mangrove-vegetated shores in wet and dry seasons. In addition, we estimated seasonal erosion/accretion rates for 2.4 years and seedling density in two zones from the forest edge with contrasting wave exposure. Regression analysis was carried out to explain sediment properties and erosion rate variance in response to the vegetation volume that opposes wave energy and to explain erosion rates in response to wave energy. Mangrove-vegetated shores reduced erosion rates from 3 to 15 times in the two sites with higher wave energy, while the vegetated site with the lowest wave energy experienced accretion compared to minor erosion along the unvegetated shore. Shores with greater Rhizophora mangle basal areas and vegetation volumes favored deposition of particles with low settling rates, different sediment classes, reduced erosion rates, and increased shoreline stability. Mangrove seedling density decreased between 2 and 43 times from the low wave exposure zone to the high wave exposure zone at the forest edge in studied sites. In order to increase vegetation volume, coastal adaptation based on mangroves must limit human disturbances and facilitate epiphytic relationships with oysters.

Differential Mortality of North Atlantic Bivalve Molluscs During Harmful Algal Blooms Caused by the Dinoflagellate, Cochlodinium (a.k.a. Margalefidinium) polykrikoides

Abstract: Blooms of the dinoflagellate, Cochlodinium (akaMargalefidinium) polykrikoides, have had deleterious effects on marine life across the Northern Hemisphere and, since the early 1990s, have become more frequent and widespread While the toxic effects of C polykrikoides have been well-described for finfish, the effects on bivalve molluscs are poorly understood, particularly in ecosystem and aquaculture settings The purpose of this study was to characterize the comparative effects of C polykrikoides blooms on North Atlantic bivalves and to identify the environmental factors that influence its toxic effects The growth and survival of two age-classes (first- and second-year) of the northern quahog (Mercenaria mercenaria), the bay scallop (Argopecten irradians), and the eastern oyster (Crassostrea virginica) were quantified in surface deployments and at depth during annual bloom events in multiple locations across eastern Long Island (NY, USA), capturing a natural gradient in C polykrikoides In two consecutive years, scallops deployed within surface locations experienced significant mortality (75–100%) during short-term (1–2 weeks) but intense (> 15 × 104 cells mL−1) C polykrikoides blooms Conversely, scallops deployed at depth and clams and oysters deployed at either the surface or at depth were more resistant to blooms First-year oysters and scallops that survived blooms displayed significant reductions in growth rates, while clams and older scallops and oysters did not Results suggest that blooms of C polykrikoides pose significant age- and species-specific threats to native and cultured bivalve shellfish and that shellfish deployed in surface waters are at greater risk during blooms than those deployed at depth

Vertical Zonation and Niche Breadth of Tidal Marsh Plants Along the Northeast Pacific Coast

Abstract: The distribution patterns of sessile organisms in coastal intertidal habitats typically exhibit vertical zonation, but little is known about variability in zonation among sites or species at larger spatial scales. Data on such heterogeneity could inform mechanistic understanding of factors affecting species distributions as well as efforts to assess and manage coastal species and habitat vulnerability to sea-level rise. Using data on the vertical distribution of common plant species at 12 tidal marshes across the US Pacific coast, we examined heterogeneity in patterns of zonation to test whether distributions varied by site, species, or latitude. Interspecific zonation was evident at most sites, but the vertical niches of co-occurring common species often overlapped considerably. The median elevation of most species varied across marshes, with site-specific differences in marsh elevation profiles more important than differences in latitude that reflect regional climate gradients. Some common species consistently inhabited lower or higher elevations relative to other species, but others varied among sites. Vertical niche breadth varied more than twofold among species. These results indicate that zonation varies by both site and species at the regional scale, and highlight the potential importance of local marsh elevation profiles to plant vertical distributions. Furthermore, they suggest that coastal foundation species such as marsh plants may differ in their vulnerability to sea-level rise by being restricted to specific elevation zones or by occurring in narrow vertical niches.

Hydrographic and Biological Impacts of a Glacial Lake Outburst Flood (GLOF) in a Patagonian Fjord

Abstract: Glacial lake outburst floods (GLOFs) in Northern Patagonian Ice Field affecting the Baker River basin have increased their frequency in recent years. To evaluate the impact of a GLOF in the hydrography and biological components of the plankton in the Baker Fjord, we assessed the relative contributions of terrigenous versus marine plankton carbon sources to the particulate organic matter (POM) in the fjord before and after a GLOF in the austral summer 2014. We also evaluated whether terrestrial carbon brought into the fjord by the river may reach higher trophic levels via a deposit-feeding organism the juvenile pelagic Munida gregaria. Over a 10-day period, hydrographic profiles, water samples for POM, and zooplankton samples were collected daily from three stations and two depths along the fjord’s inner section. Samples of suspended POM and tissue from M. gregaria were analyzed for stable-isotope composition of carbon (δ13C). The GLOF arrival produced a thermal front in the fjord, followed by an oscillation of the pycnocline; an abrupt increase in the total organic carbon content of POM, which was attributed to terrestrial input; and a concurrent peak in the abundance of M. gregaria, suggesting an aggregation response to the GLOF. Understanding GLOF effects on local hydrography, productivity, and food web structure provides valuable insight on the potential responses of fjord ecosystems in general to climate change-induced variability. Given present climatic trends in high-latitude zones, more frequent GLOFs might be expected in Patagonian fjords and channels as well as in other high-latitude basins.

Evaluating Confidence in the Impact of Regulatory Nutrient Reduction on Chesapeake Bay Water Quality

Abstract: Excess nutrients derived from anthropogenic activity have resulted in the degradation of coastal water quality and an increase in low-oxygen and hypoxic events worldwide. In an effort to curb these impacts and restore water quality in the Chesapeake Bay, a maximum load of nutrients has been established based on a framework of regulatory standards and models. This research aims to evaluate the projected changes in water quality resulting from the implementation of these nutrient reductions by applying the regulatory methodology to two different models that have been previously shown to have similar model skill. Results demonstrate that although the two models differ structurally and produce a different degree of absolute change, they project a similar relative improvement in water quality along the main stem of the Chesapeake Bay and the lower reaches of the tributaries. Furthermore, the models largely agree on the attainment of regulatory water quality standards as a result of nutrient reduction, while also establishing that meeting water quality standards is relatively independent of hydrologic (wet/dry) conditions. By developing a Similarity Index that compares model results across habitat, time, and methodology, this research identifies the locations and causes of greatest uncertainty in modeled projections of water quality. Although there are specific locations and times where the models disagree, overall this research lends support and increased confidence to the appropriateness of the nutrient reduction levels and in the general impact of nutrient reduction on Chesapeake Bay water quality under current environmental conditions.

Influences of a River Dam on Delivery and Fate of Sediments and Particulate Nutrients to the Adjacent Estuary: Case Study of Conowingo Dam and Chesapeake Bay

Abstract: Dams impact the magnitude and nature of material transport through rivers to coastal waters, initially trapping much material in upstream reservoirs. As reservoirs fill, trapping decreases and bottom sediments can be scoured by high flows, increasing downstream delivery. This is the case for the Conowingo Dam, which historically has trapped much of the sediment and particulate nutrients carried by the Susquehanna River otherwise bound for Chesapeake Bay but has now reached dynamic equilibrium. While previous studies primarily focus on either delivery of river inputs or their fate in the Bay, this study synthesizes insights from field observations and modeling along the Reservoir-Bay continuum to evaluate potential impacts of infilling on Bay biogeochemistry. Results show most Susquehanna sediment and particulate nutrient loading occurs during high-flow events that occur only ~ 10% of the time. While loading during these events has increased since the late 1970s, consistent with a decreasing scour threshold for Reservoir sediments, loading during low-flow periods has declined. Loads entering the estuary are largely retained within the upper Bay but can be transported farther downstream during events. Reservoir sediments are highly refractory, and inputs of reservoir-like organic matter do not enhance modeled sediment-nutrient release in upper Bay sediments. These findings and an emerging literature highlight the Bay’s resilience to large sediment loads during events (e.g., Tropical Storm Lee in 2011), likely aided by ongoing restoration efforts and/or consistently low-moderate recent inflows (2012–2017). Thus, while events can have major short-term impacts, the long-term impact to Bay biogeochemistry is less severe.

What Promotes the Recovery of Salt Marsh Infauna After Oil Spills

Abstract: Many factors influence the rate at which biotic communities recover from environmental disasters, and a thorough understanding of these factors is needed to formulate effective mitigation strategies. The importance of foundation species, soil environmental quality, and benthic microalgae to the long-term recovery of the salt marsh infaunal community following the 2010 Deepwater Horizon oil spill was examined in northern Barataria Bay, LA, from 2011 to 2016. The community of 12 abundant taxa of meiofauna and juvenile macroinfauna began to rebound from oiling in < 2 years, but did not fully recover after 6.5 years. The pace and intensity of recovery of nematodes, copepods, most polychaetes, tanaids, juvenile bivalves, and amphipods were significantly and positively related to the recovery of Spartina alterniflora and benthic microalgae. However, total petroleum hydrocarbon concentrations remained elevated over time, and live belowground plant biomass, bulk density, dead aboveground plant biomass, and live aboveground biomass of Juncus roemerianus were not resilient, indicating that soil quality at oiled sites was insufficient to foster the recovery of the infaunal community as a whole. Recovery of the kinorhynch Echinoderes coulli, the polychaete Manayunkia aestuarina, ostracods, and juvenile gastropods was suppressed in association with these factors. Foundation species enhance salt marsh infaunal recovery by modifying habitat in the short term and improving soil quality over the longer term. Therefore, efforts to enhance the recovery of foundation species (e.g., by plantings) should benefit the recovery of microalgal primary producers and benthic consumers after oiling in salt marshes.

Short-Term Temperature Stress Results in Seagrass Community Shift in a Temperate Estuary

Abstract: Seagrass meadows are becoming increasingly stressed throughout the world, due to a variety of factors including anthropogenic nutrient and sediment loading, and extreme climatic events. Here we explore drivers of spatial and temporal community change over a 7-year period in the York River, Chesapeake Bay, VA. Historically, declines here in the dominant species, Zostera marina, have been related to a combination of short-term summertime heat stress events and chronically reduced water clarity. We quantified two temperature-driven Z. marina die-off events that resulted in a community switch from a slower growing, large climax species (Z. marina) to a faster growing, small pioneer species (Ruppia maritima) the following summer. Of the water quality variables studied here (water temperature, turbidity, and chlorophyll), water temperature was the only significant factor related to the monthly change in Z. marina cover. Our model did not find any significant drivers of change for R. maritima, though it appears to be more related to the abundance of Z. marina rather than changes to water quality. During die-off years, R. maritima is able to temporarily replace some of the lost Z. marina abundance by expanding its coverage in some areas of the bed, retreating again once Z. marina begins to recover. The extent of this replacement in terms of habitat quality is not well known and is an important area for future research, not just for seagrass beds, but for vegetated communities worldwide as their species composition is altered in response to climate change.

Is Submarine Groundwater Discharge (SGD) Important for the Historical Fish Kills and Harmful Algal Bloom Events of Mobile Bay

Abstract: Large-scale fish and crustacean kills, locally known as Jubilees, and harmful algal blooms (HABs) have been occurring in Mobile Bay (Alabama) for more than a century. In fact, the first record describing a Jubilee event in Mobile Bay during 1867 was the first ever-documented case of mass mortalities of marine animals caused by hypoxia. To evaluate the importance of submarine groundwater discharge (SGD) in the occurrence of Jubilees and HABs in Mobile Bay, a 3-year study was conducted using a multi-method approach. Significant spatial and temporal variations of SGD were revealed in the bay only by applying a combination of geochemical and shallow geophysical techniques. The development of seasonal hypoxia observed in bay waters in areas impacted by Jubilees was the result of anoxic SGD inputs, which magnitude and spatial distribution were controlled by shallow lithological heterogeneities created during the modern development of the bay. Although when compared to the river discharge SGD contributed between 0.2 (wet season) and 5% (dry season) of the total freshwater inputs to Mobile Bay, 80% of the total SGD in the bay occurred in areas ecologically impacted by hypoxia and Jubilees. In these areas, SGD comprised up to 37% of the total water inputs during the dry season, coinciding with the time of the year when Jubilees and HABs occur. In conclusion, while SGD might not be a significant source of fresh water to Mobile Bay or other estuaries worldwide, enhanced SGD caused by site-specific lithological heterogeneities can have a critical role in the development of hypoxia and ecological issues in nearshore waters.

Controls on Sediment Suspension, Flux, and Marsh Deposition near a Bay-Marsh Boundary

Abstract: The sustainability of marshes adjacent to coastal bays is driven by the exchange of sediment across the marsh-bay boundary, where edge erosion commonly leads to lateral marsh loss and enhanced vertical accretion. The timing and patterns of sediment deposition on salt marshes adjacent to larger bodies of water such as coastal bays, however, differ from those on better-studied tidal creek marshes primarily owing to the importance of wind-waves. We combined field measurements and modeling to examine controls on suspended sediment concentrations and fluxes on a tidal flat (tidal range of 1.2 m) and rates of sediment deposition on the adjacent marsh at a site on the Eastern Shore of Virginia. Suspended sediment concentrations over tidal flats were strongly controlled by waves. Yet, storm winds sufficient to drive large resuspension events often coincided with peak tidal elevations that were too low to flood the marsh, which was oriented away from the wind directions most favorable for storm surge, thereby restricting storm-driven, episodic sediment delivery to the marsh. Winds also drove wide variability in the direction of surface currents near the marsh edge when water depths were high enough to flood the marsh. Nevertheless, our results show that sediment in the upper water column over the tidal flat was effectively transported across the marsh edge during flooding tides. A sediment deposition model developed to investigate the combined effects of vegetation and wave action on depositional patterns predicted that waves displace maximum deposition inland from the marsh edge, consistent with measured deposition at the study site. Marsh deposition was sensitive to inundation frequency as well as the concentration of sediment in water flooding the marsh, underscoring the importance of nontidal controls on water surface elevation, such as meteorological effects (e.g., storm surge) and sea level rise. Whereas short-term increases in marsh inundation enhance deposition, sea level rise that results in deeper average water depths over the tidal flats decreases deposition if marsh elevation is rising in step with sea level.

Using Spatial Variability in the Rate of Change of Chlorophyll a to Improve Water Quality Management in a Subtropical Oligotrophic Estuary

Abstract: Anthropogenic eutrophication threatens numerous aquatic ecosystems across the globe. Proactive management that prevents a system from becoming eutrophied is more effective and cheaper than restoring a eutrophic system, but detecting early warning signs and problematic nutrient sources in a relatively healthy system can be difficult. The goal of this study was to investigate if rates of change in chlorophyll a and nutrient concentrations at individual stations can be used to identify specific areas that need to be targeted for management. Biscayne Bay is a coastal embayment in southeast Florida with primarily adequate water quality that has experienced rapid human population growth over the last century. Water quality data collected at 48 stations throughout Biscayne Bay over a 20-year period (1995–2014) were examined to identify any water quality trends associated with eutrophication. Chlorophyll a and phosphate concentrations have increased throughout Biscayne Bay, which is a primary indicator of eutrophication. Moreover, chlorophyll a concentrations throughout the northern area, where circulation is restricted, and in nearshore areas of central Biscayne Bay are increasing at a higher rate compared to the rest of the Bay. This suggests increases in chlorophyll a are due to local nutrient sources from the watershed. These areas are also where recent seagrass die-offs have occurred, suggesting an urgent need for management intervention. This is in contrast with the state of Florida listing of Biscayne Bay as a medium priority impaired body of water.

Two models solutions for the Douro Estuary: flood risk assessment and breakwater effects

Abstract: Estuarine floods are one of the most harmful and complex extreme events occurring in coastal environments. To predict the associated effects, characterize areas of risk, and promote population safety, numerical modeling is essential. This work performs a comparison and a combination of two 2-dimensional depth-averaged estuarine models (based on openTELEMAC-MASCARET and Delft3D hydrodynamic software packages), to develop a two-model ensemble approach that will improve forecast robustness when compared to a one-model approach. The ensemble was applied to one of the main Portuguese estuaries, the Douro river estuary, to predict the expected water levels associated with extreme river discharges in the present-day configuration with the new breakwaters. This is a region that is periodically under heavy flooding, which entails economic losses and damage to protected landscape areas and hydraulic structures. Both models accurately simulated water levels and currents for tidal- and flood-dominated validation simulations, with correlation values close to 1, RMSE below 15%, and small Bias and Skill coefficient close to 1. The two-model ensemble results revealed that the present-day estuarine mouth configuration will produce harsher effects for the riverine populations in case identical historical river floods take place. This is mainly due to the increase in the area and volume of the estuary’s sand spit related to the construction of the new breakwaters.

Hydrological Versus Biological Drivers of Nutrient and Carbon Dioxide Dynamics in a Coastal Lagoon

Abstract: Coastal lagoons are dynamic aquatic systems that are susceptible to eutrophication due to long residence times and high inputs of nutrients. We hypothesise that groundwater-derived nutrients make a significant contribution to primary production, eutrophication and carbon cycling in these systems. Here, we report results from seasonal (pre-bloom, bloom and post bloom), nutrient, pCO2 and 222Rn (a natural groundwater tracer) surveys in a coastal lagoon that frequently experiences macroalgae blooms (Avoca Lagoon, Australia). Groundwater inputs of DIN and DIP deliver the nutrient load equivalent to the entire lagoon inventory in 1.6 to 3.5 days and 1.7 and 6 days respectively, indicating groundwater was a major source of inorganic nutrients to the system. Lagoon pCO2 displayed significant spatial and temporal variability, with the average pCO2 shifting from 1717 μatm during pre-bloom, to 137 μatm during the bloom and 3056 μatm post bloom. Radon-222 displayed a significant positive relationship with dissolved inorganic nitrogen (DIN) and pCO2 during the pre-bloom period. This suggests a hydrological dominance of surface water DIN and pCO2 during the pre-bloom period. During the bloom period, DIN displayed a positive relationship with pCO2 and negative relationship with dissolved oxygen, indicating a strong biological control over the nutrient pool. Phosphate did not appear to be controlled by either groundwater inputs, or ecosystem metabolism throughout the study. While groundwater discharge stimulated primary production through nutrient inputs (thus reducing surface water CO2), it also directly delivered significant quantities of CO2 to surface waters. The net effect of groundwater inputs of nutrients and dissolved carbon into the lagoon was a stimulation of CO2 fluxes to the atmosphere.

Factors Controlling Seasonal Phytoplankton Dynamics in the Delaware River Estuary: an Idealized Model Study

Abstract: Phytoplankton biomass in estuaries is controlled by complex biological and chemical processes that control growth and mortality, and physical processes that control transport and dilution. The effects of these processes on phytoplankton blooms were systematically analyzed, focusing on identifying the dominant controlling factors out of river-induced flushing, tidal dispersion, nutrient limitation, and light limitation. To capture the physical processes related to flow and sediment dynamics, we used the idealized width-averaged iFlow model. The model was extended with a nutrient-phytoplankton module to capture the essential biological-chemical processes. The model was applied to the Delaware River Estuary for the productive months of March to November. Model results were compared with field observations. It was found that phytoplankton blooms cannot form in the lower bay due to tidal dispersion, as water from the estuary and coastal ocean mix in early spring, and due to local effects of nitrogen limitation in summer. In the middle to upper bay, sediment-induced deterioration of the light climate limits the growth but allows for blooms in the mid bay, while no blooms can form in the turbidity maximum zone in the upper estuary. Further upstream in the tidal river, the effects of river-induced flushing dominate in early spring and prevent bloom formation. In the summer and fall, lower river discharges and higher growth rates at higher temperatures allow blooms to form and persist. Analysis of the connectivity between mid bay and tidal river blooms showed that coastal ocean phytoplankton may contribute to mid bay blooms, but do not penetrate beyond the turbidity maximum zone.