Showing papers in "Estuaries and Coasts in 2007"

Hypoxia in the northern Gulf of Mexico: Does the science support the Plan to Reduce, Mitigate, and Control Hypoxia?

Abstract: We update and reevaluate the scientific information on the distribution, history, and causes of continental shelf hypoxia that supports the 2001 Action Plan for Reducing, Mitigating, and Controlling Hypoxia in the Northern Gulf of Mexico (Mississippi River/Gulf of Mexico Watershed Nutrient Task Force 2001), incorporating data, publications, and research results produced since the 1999 integrated assessment. The metric of mid-summer hypoxic area on the LouisianaTexas shelf is an adequate and suitable measure for continued efforts to reduce nutrients loads from the Mississippi River and hypoxia in the northern Gulf of Mexico as outlined in the Action Plan. More frequent measurements of simple metrics (e.g., area and volume) from late spring through late summer would ensure that the metric is representative of the system in any given year and useful in a public discourse of conditions and causes. The long-term data on hypoxia, sources of nutrients, associated biological parameters, and paleoindicators continue to verify and strengthen the relationship between the nitratenitrogen load of the Mississippi River, the extent of hypoxia, and changes in the coastal ecosystem (eutrophication and worsening hypoxia). Multiple lines of evidence, some of them representing independent data sources, are consistent with the big picture pattern of increased eutrophication as a result of long-term nutrient increases that result in excess carbon production and accumulation and, ultimately, bottom water hypoxia. The additional findings arising since 1999 strengthen the science supporting the Action Plan that focuses on reducing nutrient loads, primarily nitrogen, through multiple actions to reduce the size of the hypoxic zone in the northern Gulf of Mexico.

The primacy of top-down effects in shallow benthic ecosystems

Abstract: Individual scientists, scientific organizations, and government agencies have all concluded that eutrophication is among the most detrimental of all human activities in coastal ecosystems; very large amounts of funding have been earmarked to study the negative consequences of nutrient pollution. Most studies of eutrophication have been conducted long after the numbers and diversity of larger marine consumers were dramatically reduced by centuries of intense harvesting. It is now understood that these once abundant predators played pivotal roles in regulating ecosystem structure and function, and that the widespread overharvesting of large consumers can trigger indirect effects that alter species compositions in ways that are very similar to those reported to result from eutrophication. All of this suggests that we should reevaluate whether the many negative effects attributed to eutrophication are actually a result of nutrient additions or whether they may be the result of the indirect effects of dramatically altered coastal food webs. In this essay, we review experimental assessments of the degree to which changes in consumer abundances have indirectly altered the structure of benthic ecosystems in coastal waters, and on the relative importance of top-down and bottom-up effects on coral reefs, rocky shores, and seagrass meadows. We find that the evidence clearly indicates that indirect consumer effects are the primary drivers of coastal benthic ecosystem structure and function.

Can oyster restoration reverse cultural eutrophication in Chesapeake Bay

Abstract: We investigated the hypothesis that effects of cultural eutrophication can be reversed through natural resource restoration via addition of an oyster module to a predictive eutrophication model. We explored the potential effects of native oyster restoration on dissolved oxygen (DO), chlorophyll, light attenuation, and submerged aquatic vegetation (SAV) in eutrophic Chesapeake Bay. A tenfold increase in existing oyster biomass is projected to reduce system-wide summer surface chlorophyll by approximately 1 mg m−3, increase summer-average deep-water DO by 0.25 g m−3, add 2100 kg C (20%) to summer SAV biomass, and remove 30,000 kg d−1 nitrogen through enhanced denitrification. The influence of osyter restoration on deep extensive pelagic waters is limited. Oyster restoration is recommended as a supplement to nutrient load reduction, not as a substitute.

Wind-driven sediment suspension controls light availability in a shallow coastal lagoon

Abstract: Light availability is critically important for primary productivity in coastal systems, yet current research approaches may not be adequate in shallow coastal lagoons. Light attenuation in these systems is typically dominated by suspended sediment, while light attenuation in deeper estuaries is often dominated by phytoplankton. This difference in controls on light attenuation suggests that physical processes may exert a greater influence on light availability in coastal lagoons than in deeper estuaries. Light availability in Hog Island Bay, a shallow coastal lagoon on the eastern shore of Virginia, was determined for a summer and late fall time period with different wind conditions. We combined field measurements and a process-based modeling approach that predicts sediment suspension and light availability from waves and currents to examine both the variability and drivers of light attenuation. Total suspended solids was the only significant predictor of light attenuation in Hog Island Bay. Waves and currents in Hog Island Bay responded strongly to wind forcing, with bottom stresses from wind driven waves dominant for 60% of the modeled area for the late fall period and 24% of the modeled area for the summer period. Higher wind speeds in late fall than in summer caused greater sediment suspension (41 and 3 mg l−1 average, respectively) and lower average (spatial and temporal) downwelling light availability (32% and 55%, respectively). Because of the episodic nature of wind events and the spatially variable nature of sediment suspension, conventional methods of examining light availability, such as fair-weather monitoring or single in situ recorders, do not adequately represent light conditions for benthic plants.

Characterization of Nutrient, Organic Carbon, and Sediment Loads and Concentrations from the Mississippi River into the Northern Gulf of Mexico

Abstract: We synthesize and update the science supporting the Action Plan for Reducing, Mitigating, and Controlling Hypoxia in the Northern Gulf of Mexico (Mississippi River/Gulf of Mexico Watershed Nutrient Task Force 2001) with a focus on the spatial and temporal discharge and patterns of nutrient and organic carbon delivery to the northern Gulf of Mexico, including data through 2006. The discharge of the Mississippi River watershed over 200 years varies but is not demonstrably increasing or decreasing. About 30% of the Mississippi River was shunted westward to form the Atchafalaya River, which redistributed water and nutrient loads on the shelf. Data on nitrogen concentrations from the early 1900s demonstrate that the seasonal and annual concentrations in the lower river have increased considerably since then, including a higher spring loading, following the increase in fertilizer applications after World WarII. The loading of total nitrogen (TN) fell from 1990 to 2006, but the loading of total phosphorus (TP) has risen slightly, resulting in a decline in the TN:TP ratios. The present TN:TP ratios hover around an average indicative of potential nitrogen limitation on phytoplankton growth, or balanced growth limitation, but not phosphorus limitation. The dissolved nitrogen:dissolved silicate ratios are near the Redfield ratio indicative of growth limitations on diatoms. Although nutrient concentrations are relatively high compared to those in many other large rivers, the water quality in the Mississippi River is not unique in that nutrient loads can be described by a variety of land-use models. There is no net removal of nitrogen from water flowing through the Atchafalaya basin, but the concentrations of TP and suspended sediments are lower at the exit point (Morgan City, Louisiana) than in the water entering the Atchafalaya basin. The removal of nutrients entering offshore waters through diversion of river water into wetlands is presently less than 1% of the total loadings going directly offshore, and would be less than 8% if the 10,093 km2 of coastal wetlands were successfully engineered for that purpose. Wetland loss is an insignificant contribution to the carbon loading offshore, compared to in situ marine production. The science-based conclusions in the Action Plan about nutrient loads and sources to the hypoxic zone off Louisiana are sustained by research and monitoring occurring in the subsequent 10 years.

Threshold effects of coastal urbanization on Phragmites australis (common reed) abundance and foliar nitrogen in Chesapeake Bay.

Abstract: The invasion of North American tidal marshes byPhragmites australis, or common reed, is a large-scale ecological problem that has been primarily studied at small spatial scales. Previous local-scale studies have provided evidence that the expansion ofPhragmites is facilitated by disturbance and increased nitrogen (N) associated with agricultural and urban-suburban (developed) land uses along wetland-upland borders. We tested the generality of previous findings across a larger spatial scale and wider range of environmental conditions in Chesapeake Bay, the largest estuarine ecosystem in the USA. We sampled 90 tidal wetlands nested within 30 distinct subestuarine watersheds and examined the relationship between land use andPhragmites abundance and foliar N, an indicator of nitrogen availability. We estimated land use adjacent to wetland borders and within subestuary watersheds and explored the importance of spatial proximity by weighting land use by its distance from the wetland border or subestuary shoreline, respectively. Regression tree and changepoint analyses revealed thatPhragmites abundance sharply increased in almost every wetland where development adjacent to borders exceeded 15%. Where development was 14–22% shoreline development, the same level of development that corresponded to high levels of invasion. Our results suggest that development near wetlands is at least partially responsible for patterns of invasion across Chesapeake Bay. Larger-scale phenomena, such as nitrogen pollution at the watershed-subestuary scale, also may be facilitating invasion. Urbanization near coastlines appears to play an important role in the invasion success ofPhragmites in coastal wetlands of Chesapeake Bay and probably much of eastern North America.

Interactive Effects of Hydrology and Salinity on Oligohaline Plant Species Productivity: Implications of Relative Sea-level Rise

Abstract: Sea-level rise is anticipated to alter hydrologic and salinity regimes of coastal wetlands. We conducted a mesocosm experiment to determine species-level responses to 12 sea-level rise scenarios. Both hydrologic regime (−10, +5, and +20 cm flooding depth) and salinity level (fresh, 2‰, 4‰ and 6‰) were interactively manipulated. Within these various sea-level rise scenarios, we sought to determine the effects of hydrologic regime, salinity level, and the interaction of these two stresses on the productivity ofPanicum hemitomon, Sagittaria lancifolia, andSpartina patens, which are dominant macrophytes of fresh, intermediate, and brackish marsh types, respectively, in coastal Louisiana and the southeastern coastal plain. We found that altered hydrologic regimes and increased salinity levels differentially affected edaphic conditions and species-level productivity. Increases in flooding depth were most detrimental toS. patens. Salinity levels greater than 4‰ resulted in mortality ofP. hemitomon, and salinity levels of 6‰ resulted in reduced growth and eventual death, ofS. lancifolia. The effects of elevated salinity levels onP. hemitomon andS. lancifolia were exacerbated when coupled with increased flooding levels. Although soil organic matter was shown to increase in all vegetative conditions, increases were dependent upon the productivity of the species under the different hydrologic regimes and salinity levels withP. hemitomon displaying tremendous potential to increase soil organic matter under fresh conditions, especially when coupled with moderate flooding. The results of this study indicate that as plant communities are subjected to long-term changes in hydrology and salinity levels, community productivity and sustainability ulimately will be determined by species-level tolerances in conjunction with species interactions.

A nearshore model to investigate the effects of seagrass bed geometry on wave attenuation and suspended sediment transport

Abstract: The effects of seagrass bed geometry on wave attenuation and suspended sediment transport were investigated using a modified Nearshore Community Model (NearCoM). The model was enhanced to account for cohesive sediment erosion and deposition, sediment transport, combined wave and current shear stresses, and seagrass effects on drag. Expressions for seagrass drag as a function of seagrass shoot density and canopy height were derived from published flume studies of model vegetation. The predicted reduction of volume flux for steady flow through a bed agreed reasonably well with a separate flume study. Predicted wave attenuation qualitatively captured seasonal patterns observed in the field: wave attenuation peaked during the flowering season and decreased as shoot density and canopy height decreased. Model scenarios with idealized bathymetries demonstrated that, when wave orbital velocities and the seagrass canopy interact, increasing seagrass bed width in the direction of wave propagation results in higher wave attenuation, and increasing incoming wave height results in higher relative wave attenuation. The model also predicted lower skin friction, reduced erosion rates, and higher bottom sediment accumulation within and behind the bed. Reduced erosion rates within seagrass beds have been reported, but reductions in stress behind the bed require further studies for verification. Model results suggest that the mechanism of sediment trapping by seagrass beds is more complex than reduced erosion rates alone; it also requires suspended sediment sources outside of the bed and horizontal transport into the bed.

A Review of Water Column Processes Influencing Hypoxia in the Northern Gulf of Mexico

Abstract: In this review, we use data from field measurements of biogeochemical processes and cycles in the Mississippi River plume and in other shelf regions of the northern Gulf of Mexico to determine plume contributions to coastal hypoxia. We briefly review pertinent findings from these process studies, review recent mechanistic models that synthesize these processes to address hypoxia-related issues, and reinterpret current understanding in the context of these mechanistic models. Some of our conclusions are that both nitrogen and phosphorus are sometimes limiting to phytoplankton growth; respiration is the main fate of fixed carbon in the plume, implying that recycling is the main fate of nitrogen; decreasing the river nitrate loading results in less than a 1:1 decrease in organic matter sinking from the plume; and sedimenting organic matter from the Mississippi River plume can only fuel about 23% of observed coastal hypoxia, suggesting significant contributions from the Atchafalaya River and, possibly, coastal wetlands. We also identify gaps in our knowledge about controls on hypoxia, and indicate that some reinterpretation of our basic assumptions about this system is required. There are clear needs for improved information on the sources, rates, and locations of organic matter sedimentation; for further investigation of internal biogeochemical processes and cycling; for improved understanding of the rates of oxygen diffusion across the pycnocline; for identification and quantification of other sources of organic matter fueling hypoxia or other mechanisms by which Mississippi River derived organic matter fuels hypoxia; and for the development of a fully coupled physical-biogeochemical model.

Morphological variation and habitat modification are strongly correlated for the autogenic ecosystem engineer Spartina anglica (common cordgrass)

Abstract: We explored to what extent morphological variation and habitat modification are correlated for an autogenic ecosystem engineer, which is an organism that modifies its habitat via its own physical structures. The intertidal salt marsh species Spartina anglica is well known for its capacity to enhance sediment accretion within its canopy by reducing hydrodynamic energy. Sediment accretion is favorable to Spartina, as it reduces inundation stress, enhances soil drainage, and enhances nutrient availability. Shoot density and clonal architecture showed a large variation that was strongly correlated with the marsh elevation and sediment type. This correlation showed that at the lowest elevations at the muddy site, Spartina tussocks had the highest shoot density, which is known to be favorable for sediment accretion by reduction of hydrodynamic energy. There was also a strong positive correlation between the amount of sediment that accumulated within a tussock and gully formation around that Spartina tussock. The tussocks at the lowest elevations at the muddy site had the lowest lateral tussock growth. At the highest elevations at the sandy site, stem densities were lower and there was a relatively high rate of clonal expansion and marsh formation. At this location, we also observed tussock mortality due to erosion of several of these openly structured tussocks. Based on the observed correlations, we hypothesize that the morphology of Spartina represents trade-offs between the capacity to maximize habitat modification through sediment trapping by having a high shoot density versus the capacity to maximize clonal expansion by spreading shoots widely and the capacity of maximal clonal expansion by spreading shoots widely versus the risk of tussock mortality due to insufficient modification of the habitat that makes the tussock vulnerable to erosion. Our results indicated that morphological variation and habitat modification are strongly correlated for the autogenic ecosystem engineer S. anglica.

Forecasting Gulf's Hypoxia: The Next 50 Years?

Abstract: This review discusses the use of hypoxia models in synthesizing the knowledge about the causes of Gulf of Mexico hypoxia, predicting the probable consequences of management actions, and building a consensus about the management of hypoxia. It also offers suggestions for future efforts related to simulating and forecasting Gulf hypoxia. The existing hypoxia models for the northern Gulf of Mexico range from simple regression models to complex three-dimensional simulation models, and they capture very different aspects of the physics, chemistry, and biology of this region. Several of these models were successfully calibrated to observations relevant for their process formulations and spatial-temporal scales. Available model results are compared to reach the consensus that large-scale hypoxia probably did not begin in the Gulf of Mexico until the mid 1970s, and that the 30% nitrogen load reduction that is called for by the Action Plan may not be sufficient to achieve its goal. The present models results suggest that a 40-45% reduction in riverine nitrogen load may be necessary to achieve the desired reduction in the areal extent of hypoxia. These model results underscore the importance of setting this goal as a running average because of significant interannual variability. Caution is raised for setting resource management goals without considering the long-term consequences of climate variability and change.

Variability of continental riverine freshwater and nutrient inputs into the north sea for the years 1977-2000 and its consequences for the assessment of eutrophication

Abstract: We determined the monthly and annual riverine freshwater, nitrogen (N) and phosphorus (P) loading into the North Sea from Belgium, The Netherlands, and Germany for the years 1977-2000. An average of 133 km 3 yr 21 of the 309 km 3 yr 21 precipitation into the watershed is carried by the rivers into the sea. Total freshwater discharge fluctuates with a strong 6- 7 yr periodicity, is strongly correlated with precipitation, and exhibits a slight long-term decrease. The temporal changes of regional patterns of precipitation lead to changing ratios of annual discharge of the western rivers compared to the eastern rivers, varying between 2.2 and 3.5. The long-term oscillations in discharge were more pronounced as discharge increased. The annual means of total and dissolved inorganic N and P loads were estimated to be 722 and 582 kt N yr 21 and 48 and 26 kt Py r 21 , respectively. The monthly N loads were much more strongly correlated with discharge, compared to the monthly P loads. Total N and P as well as dissolved inorganic N also demonstrated a 6-7 yr periodicity. The annual N loads decreased by about 17 kt N yr 21 from 1977 to 2000. The total phosphorus and phosphate loads decreased from about 80 and 50 kt P yr 21 in the 1980s to 25 and 12 kt P yr 21 , respectively, in the 1990s. The western rivers contributed the major part of the nutrient loads. The long-term oscillations in their nutrient loads were much more pronounced, compared to the eastern rivers. The area-specific loading rates estimated for all rivers are comparable to earlier estimates using shorter data records, smaller sample sizes, and a less complete watershed monitoring program. The monthly and annual average N:P ratios and their variability increased considerably for individual rivers during the study interval. These results confirm that the water quality of European continental rivers is strongly influenced by intense land use. They demonstrate the necessity for using long time series monitoring results to assess change and evaluate the effects of climate change on the North Sea coastal ecosystems, using ecosystem models on decadal time scales.

Phytoplankton production and nutrient distributions in a subtropical estuary: Importance of freshwater flow

Abstract: The relationships between phytoplankton productivity, nutrient distributions, and freshwater flow were examined in a seasonal study conducted in Escambia Bay, Florida, USA, located in the northeastern Gulf of Mexico. Five sites oriented along the salinity gradient were sampled 24 times over the 28-mo period from 1999 to 2001. Water column profiles of temperature and salinity were measured along with surface chlorophyll and surface inorganic nutrient concentrations. Primary productivity was measured at 2 sites on 11 dates, and estimated for the remaining dates and sites using an empirical regression model relating phytoplankton net production to the product of chlorophyll, euphotic zone depth, and daily solar insolation. Freshwater flow into the system varied markedly over the study period with record low flow during 2000, a flood event in March 2001, and subsequent resumption of normal flow. Flushing times ranged from 1 d during the flood to 20 d during the drought. Freshwater input strongly affected surface salinity distributions, nutrient flux, chlorophyll, and primary productivity. The flood caused high turbidity and rapid flushing, severely reducing phytoplankton production and biomass accumulation. Following the flood, phytoplankton biomass and productivity sharply increased. Analysis of nutrient distributions suggested Escambia Bay phytoplankton alternated between phosphorus limitation during normal flow and nitrogen limitation during low flow periods. This study found that Escambia Bay is a moderately productive estuary, with an average annual integrated phytoplankton production rate of 290 g C m−2 yr−1.

Oceanography of Glacier Bay, Alaska: Implications for biological patterns in a glacial fjord estuary

Abstract: Alaska, U.S.A, is one of the few remaining locations in the world that has fjords that contain temperate idewater glaciers. Studying such estuarine systems provides vital information on how deglaciation affects oceanographic onditions of fjords and surrounding coastal waters. The oceanographic system of Glacier Bay, Alaska, is of particular interest ue to the rapid deglaciation of the Bay and the resulting changes in the estuarine environment, the relatively high oncentrations of marine mammals, seabirds, fishes, and invertebrates, and the Bay’s status as a national park, where ommercial fisheries are being phased out. We describe the first comprehensive broad-scale analysis of physical and iological oceanographic conditions within Glacier Bay based on CTD measurements at 24 stations from 1993 to 2002. easonal patterns of near-surface salinity, temperature, stratification, turbidity, and euphotic depth suggest that freshwater nput was highest in summer, emphasizing the critical role of glacier and snowmelt to this system. Strong and persistent tratification of surface waters driven by freshwater input occurred from spring through fall. After accounting for seasonal nd spatial variation, several of the external physical factors (i.e., air temperature, precipitation, day length) explained a large mount of variation in the physical properties of the surface waters. Spatial patterns of phytoplankton biomass varied hroughout the year and were related to stratification levels, euphotic depth, and day length. We observed hydrographic atterns indicative of strong competing forces influencing water column stability within Glacier Bay: high levels of freshwater ischarge promoted stratification in the upper fjord, while strong tidal currents over the Bay’s shallow entrance sill enhanced ertical mixing. Where these two processes met in the central deep basins there were optimal conditions of intermediate tratification, higher light levels, and potential nutrient renewal. These conditions were associated with high and sustained hlorophylla levels observed from spring through fall in these zones of the Bay and provide a framework for understanding he abundance patterns of higher trophic levels within this estuarine system.

Temporal variability in summertime bottom hypoxia in shallow areas of Mobile Bay, Alabama

Abstract: This paper addresses temporal variability in bottom hypoxia in broad shallow areas of Mobile Bay, Alabama. Time-series data collected in the summer of 2004 from one station (mean depth of 4 m) exhibit bottom dissolved oxygen (DO) variations associated with various time scales of hours to days. Despite a large velocity shear, stratification was strong enough to suppress vertical mixing most of the time. Bottom DO was closely related to the vertical salinity gradient (ΔS). Hypoxia seldom occurred when ΔS (over 2.5 m) was 8 psu in the absence of extreme events like hurricanes. Oxygen balance between vertical mixing and total oxygen demand was considered for bottom water from which oxygen demand and diffusive oxygen flux were estimated. The estimated decay rates at 20°C ranging between 0.175–0.322 d−1 and the corresponding oxygen consumption as large as 7.4 g O2 m−2 d−1 fall at the upper limit of previously reported ranges. The diffusive oxygen flux and the corresponding vertical diffusivity estimated for well mixed conditions range between 8.6–9.5 g O2 m−2 d−1 and 2.6–2.9 m2 d−1, respectively. Mobile Bay hypoxia is likely to be associated with a large oxygen demand, supported by both water column and sediment oxygen demands, so that oxygen supply from surface water during destratification events would be quickly exhausted to return to hypoxic conditions within a few hours to days after destratification events are terminated.

Trophic assessment in Chinese coastal systems - Review of methods and application to the Changjiang (Yangtze) Estuary and Jiaozhou Bay

Abstract: Coastal eutrophication has become one of the main threats to Chinese coastal areas during the last two decades. High nutrient loads from human activities have modified the natural background water quality in coastal water bodies, resulting in a range of undesirable effects. There is a need to assess the eutrophic level in coastal systems and to identify the extent of this impact to guide development of appropriate management efforts. Traditional Chinese assessment methods are discussed and compared with other currently-used methods, such as the Oslo-Paris Convention for the Protection of the North Sea (OSPAR) Comprehensive Procedure and Assessment of Estuarine Trophic Status (ASSETS). The ASSETS method and two Chinese methods were tested on two Chinese systems: the Changjiang (Yangtze) Estuary and Jiaozhou Bay. ASSETS is process based, and uses a pressure-state-response model based on three main indices: Influencing Factors, Overall Eutrophic Condition, and Future Outlook. The traditional methods are based on a nutrient index. ASSETS was successfully applied to both systems, classifying the Changjiang Estuary as Bad (high eutrophication) and Jiaozhou Bay as High (low eutrophication). The traditional methods led to ambiguous results, particularly for Jiaozhou Bay, due to the high spatial variability of data and a failure to assess the role of shellfish aquaculture in nutrient control. An overview of the Chinese coastal zone identifies 50 estuaries and bays that should form part of a national assessment. A comparison of methods and results suggests that ASSETS is a promising tool for evaluating the eutrophication status of these systems.

Testing the predictive power of seagrass depth limit models

Abstract: The power of equations predicting seagrass depth limit (Zc) from light extinction (K z) was tested on data on seagrass depth limits collected from the literature. The test data set comprised 424 reports of seagrass colonization depth and water transparency, including data for 10 seagrass species. This data set confirmed the strong negative relationship betweenZ c andK z. The regression equation in Duarte (1991) overestimated the realized seagrass colonization depths at colonization depths < 5 m, while there was no prediction bias above this threshold. These results indicated that seagrass colonizing turbid waters (K z 0.27 m-1) have higher apparent light requirements than those growing in clearer waters. The relationship between seagrass colonization depth and light attenuation shifts at a threshold of light attenuation of 0.27 m-1, requiring separate equations to predictZ c for seagrass growing in more turbid waters and clearer waters, and to set targets for seagrass restoration and conservation efforts.

Shifting nutrient limitation and eutrophication effects in marsh vegetation across estuarine salinity gradients

Abstract: In light of widespread coastal eutrophication, identifying which nutrients limit vegetation and the community consequences when limitation is relaxed is critical to maintaining the health of estuarine marshes. Studies in temperate salt marshes have generally identified nitrogen (N) as the primary limiting nutrient for marsh vegetation, but the limiting nutrient in low salinity tidal marshes is unknown. I use a 3-yr nutrient addition experiment in mid elevation, Spartina patens dominated marshes that vary in salinity along two estuaries in southern Maine to examine variation in nutrient effects. Nutrient limitation shifted across estuarine salinity gradients; salt and brackish marsh vegetation was N limited, while oligohaline marsh vegetation was co-limited by N and phosphorus (P). Plant tissue analysis of S. patens showed plants in the highest salinity marshes had the greatest percent N, despite N limitation, suggesting that N limitation in salt marshes is partially driven by a high demand for N to aid in salinity tolerance. Fertilization had little effect on species composition in monospecific S. patens stands of salt and brackish marshes, but N + P treatments in species-rich oligohaline marshes significantly altered community composition, favoring dominance by high aboveground producing plants. Eutrophication by both N and P has the potential to greatly reduce the characteristic high diversity of oligohaline marshes. Inputs of both nutrients in coastal watersheds must be managed to protect the diversity and functioning of the full range of estuarine marshes.

Littoral Fish Assemblages of the Alien-dominated Sacramento- San Joaquin Delta, California, 1980-1983 and 2001-2003

Abstract: We analyzed monthly boat electrofishing data to characterize the littoral fish assemblages of five regions of the Sacramento-San Joaquin Delta (northern, southern, eastern, western, and central), California, during two sampling periods, 1980-1983 (1980s) and 2001-2003 (2000s), to provide information pertinent to the restoration of fish populations in this highly altered estuary. During the 1980s, almost 11,000 fish were captured, including 13 native species and 24 alien species. During the 2000s, just over 39,000 fish were captured, including 15 native species and 24 alien species. Catch per unit effort (CPUE) of total fish, alien fish, and centrarchid fish were greater in the 2000s compared with the 1980s, largely because of increased centrarchid fish CPUE. These differences in CPUE were associated with the spread of submerged aquatic vegetation (SAV), particularly an alien aquatic macrophyte Egeria densa. Native fish CPUE declined from the 1980s to the 2000s, but there was no single factor that could explain the decline. Native fish were most abundant in the northern region during both sampling periods. Nonmetric multidimensional scaling indicated similar patterns of fish assemblage composition during the two sampling periods, with the northern and western regions characterized by the presence of native species. The separation of the northern and western regions from the other regions was most distinct in the 2000s. Our results suggest that native fish restoration efforts will be most successful in the northern portion of the Delta. Management decisions on the Delta should include consideration of possible effects on SAV in littoral habitats and the associated fish assemblages and ecological processes.

Spatial Structure of Hydrography and Flow in a Chilean Fjord, Estuario Reloncavi´

Abstract: Underway current velocity profiles were combined with temperature and salinity profiles at fixed stations to describe tidal and subtidal flow patterns in the middle of the northernmost Chilean fjord, Estuario Reloncavi. This is the first study involving current velocity measurements in this fjord. Reloncavi fjord is 55 km long, 2 km wide, and on average is 170 m deep. Measurements concentrated around a marked bend of the coastline, where an 8-km along-fjord transect was sampled during a semidiurnal tidal cycle in March 2002 and a 2-km cross-fjord transect was occupied, also during a semidiurnal cycle, in May 2004. The fjord hydrography showed a relatively thin ( 4 kg m−3 per meter of depth. Below this thin layer, the water was relatively homogeneous. Semidiurnal tidal currents had low amplitudes (<10 cm s−1) that allowed the persistence of a surface front throughout the tidal cycle. The front oscillated with a period of ca. 2.5 h and showed excursions of 2 km. The front oscillations could have been produced by a lateral seiche that corresponds to the natural period of oscillation across the fjord. This front could have also caused large (2 h) phase lags in the semidiurnal tidal currents, from one end of the transect to the other, within the buoyant layer. Tidal phases were relatively uniform underneath this buoyant layer. Subtidal flows showed a 3-layer pattern consisting of a surface layer (8 m thick, of 5 cm s−1 surface outflow), an intermediate layer (70 m thick, of 3 cm s−1 net inflow), and a bottom layer (below 80 m depth, of 3 cm s−1 net outflow). The surface outflow and, to a certain extent, the inflow layer were related to the buoyant water interacting with the ambient oceanic water. The inflowing layer and the bottom outflow were attributed to nonlinear effects associated with a tidal wave that reflects at the fjord's head. The weak subtidal currents followed the morphology of the bend and caused downwelling on the inside and upwelling on the outside part of the bend.

Water depth modifies relative predation risk for a motile fish taxon in Bahamian tidal creeks

Abstract: We evaluated the influence of water depth on relative predation risk for mojarra (Eucinostomus spp.) in six tidal creeks on Andros Island, Bahamas. Relative predation risk was determined using a tethering protocol combined with underwater visual census. In one experiment, we found that relative predation risk increased predictably with water depth (r2 = 0.83), and survival of tethered mojarra decreased with water depth (r2 = 0.71). We identified three depth zones containing differing levels of predation threat: refugia (0–19 cm), transition (20–69 cm), and predation (> 70 cm). Predation on mojarra rarely occurred within the refugia zone (2% eaten) and always in the predation zone (100% eaten). Additional factors not examined in this study (e.g., mangrove complexity, predator density) likely drive variability of relative predation risk within the transition zone. In a second experiment, we directly examined influence of water depth on relative predation risk at fixed locations from high tide to low tide in a single creek. Mean relative predation risk was significantly higher during higher tides at deeper water depths. Results provide experimental evidence that exploitation of shallow water refugia by motile prey can significantly reduce predation risk. We expect the distribution of motile fishes is at least partially influenced by spatially-dynamic shallow water refugia.

Hydroacoustics as a tool for assessing fish biomass and size distribution associated with discrete shallow water estuarine habitats in Louisiana

Abstract: We developed a relative index of fish biomass and size distribution in ultra-shallow waters (< 2 m) of Barataria Bay, Louisiana, based on the comparison of horizontal hydroacoustic data with gill net and push trawl catches in an effort to understand the role that habitat plays in both fish biomass and distribution. Exclosure net experiments indicated that the contribution of acoustic backscattering from sources other than fishes were negligible. Split-beam transducer, gill net, and push trawl sampling were conducted concurrently in Barataria Bay to provide information on fish composition and length distributions and for comparisons among gear types. Results suggest that acoustic fish biomass was generally higher in the low salinity stations and lower at the high salinity stations, at least in March 2004. We observed a greater mean length of fishes associated with oyster shell habitats when compared to adjacent sand-mud habitats. This paper demonstrates the utility of hydroacoustics as a tool to quantify relative acoustic fish biomass and size distribution associated with common estuarine habitats in ultra-shallow waters. This study also illustrates the potential of using acoustics for augmenting traditional sampling procedures.

Mechanical Shredding of Water Hyacinth (Eichhornia crassipes): Effects on Water Quality in the Sacramento-San Joaquin River Delta, California

Abstract: Management actions to control invasive aquatic species can have significant ecosystem-scale effects. We evaluated the water chemistry and nutrient effects of mechanical shredding to control water hyacinth (Eichhornia crassipes) in an agricultural slough and a tidal wetland on the Sacramento-San Joaquin River Delta, California. Shredding was conducted with two types of shredder boats in fall of 2003 and another boat in spring of 2004. Shredding measurably affected water quality, but specific effects varied as a function of shredding site and season. Significant increases were observed for total Kjeldahl nitrogen and total phosphorus for all experiments. Dissolved oxygen effects varied by site, decreasing after shredding at the agricultural slough but increasing at the tidal wetland. The increase in dissolved oxygen likely resulted from tidal incursions from the adjacent river. A year-long time series of dissolved oxygen data indicated a negative relationship between hyacinth abundance and dissolved oxygen concentrations. Hyacinth contained similar tissue concentrations of mercury to underlying sediments, suggesting that plant harvesting could aid mercury remediation efforts. Simple mass calculations indicated that Delta-wide shredding operations could cause between 0.1% and 9.6% increases in the overall abundance of carbon, nitrogen, and phosphorus in the Delta water column. Results suggest that local effects of management actions to control invasive aquatic plants will vary widely as a function of site-specific hydrology, but that estuary-wide effects would be limited.

Modeling of the Cape Fear River Estuary plume

Abstract: The Environmental Fluid Dynamic Code, an estuarine and coastal ocean circulation model, is used to simulate the distribution of the salinity plume in the vicinity of the mouth of the Cape Fear River Estuary, North Carolina. The individual and coupled effects of the astronomical tides, river discharge, and atmospheric winds on the spatial and temporal distributions of coastal water levels and the salinity plume were investigated. These modeled effects were compared with water level observations made by the National Oceanic and Atmospheric Administration and salinity surveys conducted by the Coastal Ocean Research and Monitoring Program. Model results and observations of salinity distributions and coastal water level showed good agreement. The simulations indicate that strong winds tend to reduce the surface plume size and distort the bulge shape near the estuary mouth due to enhanced wind-induced surface mixing. Under normal discharge conditions, tides, and light winds, the southward outwelling plume veers west. Relatively moderate winds can mechanically reverse the flow direction of the plume. Under conditions of weak to moderate winds the water column does not mix vertically to the bottom, while in strong wind cases the plume becomes vertically well mixed. Under conditions of high river discharge the plume increases in size and reaches the bottom. Vertical mixing induced by strong spring tides can also enable the plume to reach the bottom.

Long-term declines in two apex predators, bull sharks ( Carcharhinus leucas ) and alligator gar ( Atractosteus spatula ), in Lake Pontchartrain, an oligohaline estuary in southeastern Louisiana

Abstract: We analyzed historic and current fishery independent data to determine if the abundance of two apex predators, bull sharks (Carcharhinus leucas) and alligator gar (Atractosteus spatula), in Lake Pontchartrain had changed significantly over the last half century. Lake Pontchartrain is an environmentally degraded oligohaline estuary in southeastern Louisiana that has experienced considerable changes in fish assemblage composition over this interval. Using gillnet, beach seine, and trawl data collected during three time periods (1953–1955, 1977–1978, and 1996–2005), we analyzed trends in abundance forC. leucas andA. spatula using generalized linear models with a negative binomial error structure and a log link. Lake Pontchartrain data were divided into four spatial locations (northwest, northeast, southwest, southeast) since each region represents a unique combination of anthropogenic and natural influences that could affect catches. For each species and gear type, we produced log-likelihood profiles for the instantaneous rate of change in relative abundance through time. Raw catches were generally lower for both species in the later surveys.C. leucas were not captured in beach seines since the 1950s andA. spatula were rarely captured in trawls or seines since the 1970s. Likelihood profiles of changes in abundance forC. leucas andA. spatula showed very large declines in both species since 1953.C. leucas declined by 98.6% (95% CI: 73.4–99.9%) in gillnets and became functionally extirpated in beach seines with a decline of 99.9% (95% CI: 23–99.9%). Among all gears,C. leucas declined by the same rate as in gillnets. The decline inA. spatula was also large with a decrease of 98.6% (95% CI: 73.4–99.9%) in beach seines and a decline of 99.2% (95% CI: 54.8–99.9%) in trawls since 1953. Catches ofA. spatula in gillnets did not show a significant change over the study period. The continued decline of these two apex predators could seriously affect efforts to restore this degraded estuarine ecosystem.

Seasonal movement and resource-use patterns of resident horseshoe crab (Limulus polyphemus) populations in a Maine, USA Estuary

Abstract: Knowledge of resource-use and movement patterns is a missing component in the development of horseshoe crab (Limulus polyphemus) management strategies. Available evidence indicates the potential for a variety of possible migratory behaviors, but the lack of high-resolution, spatial-temporal data has hindered development of a year-round profile of ranging behavior. This need was addressed in the present study by using acoustic telemetry to track the movements of adult horseshoe crabs in two subembayments (Egypt and Hog Bays) of the Taunton Bay Estuary, Maine, from June 2003 to June 2005. Estimated mean total home range sizes were 64.1 and 61.4 ha for breeding crabs tagged in Egypt and Hog Bays, respectively. We observed no horseshoe crab dispersal to areas outside of the subembayments where they were tagged, so no mixing was observed between Egypt and Hog Bay individuals despite a < 4-km separation. Observed shifts in movement patterns, resource use (subtidal versus intertidal), and vagility facilitated a profile of seasonally partitioned horseshoe crab activity, which included late April to early May post-wintering, June–July breeding, August–September pre-wintering, and October–April wintering, where space usage represented about 10% of the mean total home range size. The apparent isolation of these resident populations implies a heightened vulnerability to overexploitation and large-scale habitat alteration that might be more easily sustained by larger, more vagile populations. This work underscores the need to apply horseshoe crab conservation, research, and management efforts at scales that are appropriate to the ranging patterns of crabs, which first requires application of high-resolution methods to identify those patterns.

Does seed availability limit plant establishment during salt marsh restoration

Abstract: Patterns of seed dispersal and seed bank accumulation need to be known to predict the species that will recruit into restoration sites versus those that must be introduced. We assessed the temporal and spatial patterns of seed availability and seed accumulation on the salt marsh plain of an 8-ha resforation site, based on seedlings that emerged from tidal material, wrack, rabbit pellets, and soil samples (in controlled experiments). We compared results from the first 3 yr of restoration (2000–2002) with results from a 5-yr-old restoration and an extant marsh, all within Tijuana Estuary, California. Seed dispersal was limited for most marsh plain species. Tidal dispersal occurred mainly in winter; seedling density and richness were highest with high spring tides and after fruiting of the dominant species,Sarcocornia pacifica (>90% of emergent seedlings).S. pacifica was also common in the seedlings that emerged from soil seed banks (up to 63%) and wrack (60%), while other species common in the vegetation were present at much lower densities. Seed bank accumulation in restored sites was low and few species were abundant. Seedlings that emerged from soil samples from the youngest restoration were mostly invasive exotics (64%), those of the 5-yr restoration were mostlyS. pacifica (63%), and those from the nearby extant marsh were mostlyTriglochin concinna (70%), despite more diverse vegetation. No salt marsh seedlings emerged from rabbit pellets; all were invasives (Mesembryanthemum crystallimum andCarpobrotus edulis). Emerging seedlings were much sparser in soil from the younger restoration than from the 5-yr restoration and the extant site (32.9±5.7<4642.2±1131.5 and 4689.3±359.3 seedlings m−2, respectively). Because dispersal is limited for most species, restoring diverse vegetation will require seeding or planting. Natural recruitment could be facilitated by completing restoration by early winter, when seeds of native plants have maximum tidal dispersal.

Belowground decomposition of mangrove roots in Florida coastal everglades

Abstract: Mangrove root decomposition rates were measured by distributing mesh bags containing fine root material across six sites with different soil fertility and hydroperiod to compare ambient differences to substrate quality. Roots from a site with lower soil phosphorus concentration were used as a reference and compared to ambient roots at five other sites with increased phosphorus concentration. Four mesh bags of each root type (ambient versus reference), separated into four 10-cm replicate intervals, were buried up to 42 cm depth at each site and incubated for 250 d (initiation in May 2004). Mass loss of ambient mangrove roots was significant at all study sites and ranged from 17% to 54%; there was no significant difference with depth at any one site. Reference decomposition constants (−k) ranged from 0.0012 to 0.0018 d−1 among Taylor Slough sites compared to 0.0023–0.0028 d−1 among Shark River sites, indicating slower decomposition rates associated with lower soil phosphorous and longer flood duration. Reference roots had similar decomposition rates as ambient roots in four of the six sites, and there were no significant correlations between indices of root substrate quality and decomposition rates. Among these distinct landscape gradients of south Florida mangroves, soil environmental conditions have a greater effect on belowground root decomposition than root substrate quality.

Submersed Vegetation as Habitat for Invertebrates in the Hudson River Estuary

Abstract: We sampled epiphytic and benthic macroinvertebrates in 20 beds of submersed vegetation throughout the Hudson River estuary to assess the importance of plant beds in providing habitat for macroinvertebrates and to determine which characteristics of plant beds affected the density and composition of macroinvertebrates. Macroinvertebrate densities in plant beds were 4-5 times higher, on average, than densities in unvegetated sediments in the Hudson. The macroinvertebrate community in plant beds was dominated by chironomid midges, oligochaete worms, hydroids, gastropods, and amphipods. Many species of macroinvertebrates were found chiefly on submersed plants, showing that plant beds are important in supporting biodiversity in the Hudson. Macroinvertebrates were most numerous in beds with high plant biomass and in the interiors of beds, whereas neither bed size nor position along the length of the estuary affected macroinvertebrate density. Community composition varied strongly with position along the river (freshwater versus brackish), habitat (epiphytic versus benthic), and position within the bed (edge versus interior). Plant biomass also influenced macroinvertebrate community composition, but bed area had relatively little influence.

Effects of mesoscale physical processes on thin zooplankton layers at four sites along the west coast of the U.S.

Abstract: Within the coastal marine environment, populations of phytoplankton, zooplankton, bacteria, viruses, and aggregations of marine snow are frequently concentrated beneath the surface in discrete, vertically thin layers. Thin layers range in vertical dimension from a few centimeters to three meters, and have been observed to extend horizontally for kilometers. They appear in the water column episodically and can persist for days. We present some of the results of an investigation of four coastal sites along the west coast of the United States to assess frequency of occurrence of thin layers of zooplankton. Our study sites included coastal sites near East Sound, Washington; Cape Perpetua, Oregon; Monterey Bay, California; and Santa Barbara, California. At each site, we collected several, weeks-long time series of hydrography, current velocity, and acoustic backscatter due to mesozooplankton. Our results show that thin layers were common features at all four sites. Across all study sites, a change in the predominant physical regime, usually precipitated by a change in the wind pattern, corresponded with an absence of thin zooplankton layers. In order to make a first-order prediction about when thin layers have the possibility of occurring in a coastal environment, we found it useful to examine regional wind and circulation patterns and to determine how they affect stratification in each local environment.