Estuary In Central California Research Articles

Biological and Physical Controls on Multidecadal Acidification in a Eutrophic Estuary

AbstractEstuaries support ecologically and economically important resources that are vulnerable to ocean acidification from rising anthropogenic CO2. However, complex local processes in estuaries complicate and may disguise long‐term pH trends. For example, terrestrial nutrient runoff and coastal upwelling may exacerbate pH variability and declines. We investigated eutrophication impacts on acidification in a central California estuary, Elkhorn Slough, which receives high nutrient loads from intensive surrounding agriculture and upwelling of the California Current System. We examined drivers of acidification including nutrients, ecosystem metabolism, and upwelling by modeling pH trends over 20 years using a Generalized Additive Mixed Model at four sites from the National Estuarine Research Reserve Systemwide Monitoring Program and collected additional water samples to calculate aragonite saturation. Our models revealed acidification trends over two decades which were more pronounced near the marine inlet. Near the marine inlet, high nutrient levels and lower buffering were associated with the greatest rate of acidification in the estuary, which was four times greater than the trend from anthropogenic CO2 alone. Compared to fully tidal sites, a tidally restricted site showed diminished pH declines over time because of higher mean pH and aragonite saturation levels, but greater diel and seasonal variability associated with cycles of ecosystem metabolism and tidal range. Therefore, short‐term drops of saturation are threats to acidity in this location. The effects of enhanced seasonal cycles or long‐term trends in different zones of the estuary have implications for monitoring estuaries with a temporal frequency and scale to capture coastal acidification risks.

Use of historical isoscapes to develop an estuarine nutrient baseline.

Coastal eutrophication is a prevalent threat to the healthy functioning of ecosystems globally. While degraded water quality can be detected by monitoring oxygen, nutrient concentrations, and algal abundance, establishing regulatory guidelines is complicated by a lack of baseline data (e.g., pre-Anthropocene). We use historical carbon and nitrogen isoscapes over ~300 years from sediment cores to reconstruct spatial and temporal changes in nutrient dynamics for a central California estuary, Elkhorn Slough, where development and agriculture dramatically enhanced nutrient inputs over the past century. We found strong contrasts between current sediment stable isotopes and those from the recent past, demonstrating shifts exceeding those in previously studied eutrophic estuaries and substantial increases in nutrient inputs. Comparisons of contemporary with historical isoscapes also revealed that nitrogen sources shifted from a historical marine-terrestrial gradient with higher δ15N near the inlet to amplified denitrification at the head and mouth of the modern estuary driven by increased N inputs. Geospatial analysis of historical data suggests that an increase in fertilizer application - rather than population growth or increases in the extent of cultivated land - is chiefly responsible for increasing nutrient loads during the 20th century. This study demonstrates the ability of isotopic and stoichiometric maps to provide important perspectives on long-term shifts and spatial patterns of nutrients that can be used to improve management of nutrient pollution.

Sea otters in a California estuary: Detecting temporal and spatial dynamics with volunteer monitoring

AbstractVolunteer monitoring can support conservation of imperiled wildlife, by providing higher resolution data in space and time than those available from professional scientists. However, concerns have been raised that data collected by amateurs are inaccurate or inconsistent and thus do not allow for robust detection of spatial or temporal trends. We evaluated the rigor and value of volunteer monitoring data for one iconic wildlife species, the southern sea otter (Enhydra lutris nereis), in Elkhorn Slough estuary in central California, USA, and explored whether volunteer monitoring could provide added value to complement limited professional surveys. First, we compiled and analyzed sea otter counts taken on daily ecotourist boat trips along the estuary, and then compared temporal patterns to data collected by professional scientists tasked with monitoring this federally listed species. Second, we analyzed data on sea otter abundance, habitat use, and behavior collected by a team of trained volunteers, the Elkhorn Slough Reserve Otter Monitoring Program. Overall, we demonstrated the ability to detect important ecological patterns relevant to sea otter conservation and wetland habitat management using volunteer‐derived datasets. Long‐term trends and inter‐annual variability were similar between professional agency monitoring data and volunteer datasets. Moreover, the much higher frequency of volunteer observations allowed for seasonal and tidal dynamics to be detected that could not be revealed by less frequent professional monitoring. We found higher sea otter abundance in the estuary in spring–summer, indicating seasonality in use of the estuary. We detected differences in habitat use of the estuary between higher and lower tides, and greater frequency of foraging at low tide and in certain areas. Volunteer observations revealed fine‐scale differences in habitat use: eelgrass beds were used much more heavily than adjacent areas only a few meters away. Volunteer data can thus provide critical information about coastal habitat use and behavior that can improve conservation strategies for threatened wildlife species.

Burrowing crabs and physical factors hasten marsh recovery at panne edges.

Salt marsh loss is projected to increase as sea-level rise accelerates with global climate change. Salt marsh loss occurs along both lateral creek and channel edges and in the marsh interior, when pannes expand and coalesce. Often, edge loss is attributed to erosive processes whereas dieback in the marsh interior is linked to excessive inundation or deposition of wrack, but remains poorly understood. We conducted a two-year field investigation in a central California estuary to identify key factors associated with panne contraction or expansion. Our study explored how an abundant burrowing crab, shown to have strong negative effects on marsh biomass near creek edges, affects panne dynamics. We also explored which physical panne attributes best predicted their dynamics. To our knowledge, ours is the first study of panne dynamics in a California marsh, despite how ubiquitous pannes are as a feature of marshes in the region and how often extensive marsh dieback occurs via panne expansion. Overall, we found that pannes contracted during the study period, but with variable rates of marsh recovery across pannes. Our model incorporating both physical and biological factors explained 86% of the variation in panne contraction. The model revealed a positive effect of crab activity, sediment accretion, and a composite of depth and elevation on panne contraction, and a negative effect of panne size and distance to nearest panne. The positive crab effects detected in pannes contrast with negative effects we detected near creek edges in a previous study, highlighting the context-dependence of top-down and bioturbation effects in marshes. As global change continues and the magnitude and frequency of disturbances increases, understanding the dynamics of marsh loss in the marsh interior as well as creek banks will be critical for the management of these coastal habitats.

Rapid enhancement of multiple ecosystem services following the restoration of a coastal foundation species.

The global decline of marine foundation species (kelp forests, mangroves, salt marshes, and seagrasses) has contributed to the degradation of the coastal zone and threatens the loss of critical ecosystem services and functions. Restoration of marine foundation species has had variable success, especially for seagrasses, where a majority of restoration efforts have failed. While most seagrass restorations track structural attributes over time, rarely do restorations assess the suite of ecological functions that may be affected by restoration. Here we report on the results of two small‐scale experimental seagrass restoration efforts in a central California estuary where we transplanted 117 0.25‐m2 plots (2,340 shoots) of the seagrass species Zostera marina. We quantified restoration success relative to persistent reference beds, and in comparison to unrestored, unvegetated areas. Within three years, our restored plots expanded ~8,500%, from a total initial area of 29 to 2,513 m2. The restored beds rapidly began to resemble the reference beds in (1) seagrass structural attributes (canopy height, shoot density, biomass), (2) ecological functions (macrofaunal species richness and abundance, epifaunal species richness, nursery function), and (3) biogeochemical functions (modulation of water quality). We also developed a multifunctionality index to assess cumulative functional performance, which revealed restored plots are intermediate between reference and unvegetated habitats, illustrating how rapidly multiple functions recovered over a short time period. Our comprehensive study is one of few published studies to quantify how seagrass restoration can enhance both biological and biogeochemical functions. Our study serves as a model for quantifying ecosystem services associated with the restoration of a foundation species and demonstrates the potential for rapid functional recovery that can be achieved through targeted restoration of fast‐growing foundation species under suitable conditions.

A scientific framework for conservation aquaculture: A case study of oyster restoration in central California

The emerging field of conservation aquaculture focuses on the potential for incorporating aquaculture techniques into restoration. Extensive loss of oyster reefs worldwide has led to restoration initiatives that sometimes incorporate aquaculture, but few scientific studies of this approach have been published. We developed a scientific framework to determine whether aquaculture is an appropriate conservation tool, and applied it to Olympia oysters (Ostrea lurida) in Elkhorn Slough, an estuary in central California, USA. Over 12 years of monitoring, we documented precipitous declines in density, highlighting the need for restoration. We tracked settled oysters and found that growth and survivorship is high, showing that hatchery-raised juveniles have the potential to survive to reproductive age. No natural recruitment has occurred in the estuary in seven years, suggesting that this population is recruitment limited. Thus, we determined a need for conservation aquaculture. We produced juvenile oysters from local broodstock in a hatchery and settled them on native clam shells, which we attached to stakes to form small clusters that mimic natural biogenic habitat created by this species. We deployed these near the upper limit of the intertidal range of oysters, where oyster cover dominates over non-native fouling species. The outplanted oysters grew to adult, reproductive size within months of outplanting, and survivorship was generally high, providing the first new generation of oysters in this estuary in seven years. The science-based approach we implemented and our incorporation of traditional restoration principles of natural habitat structure and dominance by native species can serve as a model for conservation aquaculture for oysters and other species.

Multiple factors contribute to the spatially variable and dramatic decline of an invasive snail in an estuary where it was long-established and phenomenally abundant

Boom-bust dynamics of invasive species have long intrigued scientists and managers alike, but quantification of such dynamics, let alone their causes, is rare. We documented the decline of a previously prolific invasive mudsnail, Batillaria attramentaria, at Elkhorn Slough estuary in central California, USA. The mudsnail was the most abundant epibenthic invertebrate in the estuary, maintaining very high densities for many decades before declining heterogeneously throughout the estuary over the past decade, decreasing in density by three orders of magnitude at some sites. We used field and laboratory experiments to test several possible mechanisms for its demise. We show that the crab Pachygrapsus crassipes can prey heavily on Batillaria. We detected high dissolution rates of Batillaria shells, and we measured greater predation rates on tethered snails with dissolved versus intact shells. Warm water temperatures and high water levels coincided with the period of most dramatic Batillaria declines (2013–2015). Localized water impoundments appear to buffer environmental drivers of the decline because Batillaria remained abundant at sites with artificial tidal restriction, while the population crashed at one site after full tidal exchange was restored. We also investigated trematode parasite prevalence and molluscicide applications to the surrounding watershed as possible causes of mudsnail declines, but they had little explanatory power. Our findings illustrate the potential for population crashes even for long-established introduced species at pest levels of abundance, and demonstrate that such declines can exhibit spatial heterogeneity. Both of these results highlight the value of investigating population dynamics of invaders across multiple temporal and spatial scales.

The dynamics of open populations: integration of top–down, bottom–up and supply–side influences on intertidal oysters

Most communities are structured not by a single process but by some combination of top–down, bottom–up and supply–side (i.e. juvenile recruitment) factors. However, establishing how multiple processes interact remains a fundamental challenge. For example, the recruitment, growth, and mortality of estuarine species can vary along the steep and numerous environmental and biological gradients typical of these habitats, but the relative importance of those gradients is generally unknown. We took a novel approach to this question by coupling long‐term field observations of the Olympia oyster Ostrea lurida in a central California estuary with a state–space integral projection model. This approach revealed that the most parsimonious description of oyster population dynamics involved spatial variation in growth and adult mortality – but not juvenile mortality – as well as spatiotemporal variation in recruitment. These patterns match the available short‐term estimates of each of those processes from field studies, and reveal a synthetic view of oyster population dynamics. Larval recruitment has an interannual ‘boom and bust’ pattern, and during good recruitment years most larvae settle in the inner bay where water residence time is highest. Adult oyster mortality is also highest in the inner bay, where several invasive predators are abundant and lowest in the mid‐bay, where oyster growth is greatest (due to bottom–up factors), likely leading to a size refuge from native predators. Surprisingly, juvenile mortality was constant across the bay, possibly because of a lack of size refuge from native and invasive predators. Our research approach represents an important advance in disentangling the contributions of spatio–temporal variation in top–down, bottom–up and supply–side forces to the dynamics of populations with open recruitment.

Eutrophication decreases salt marsh resilience through proliferation of algal mats

Globally, many estuaries are affected by nutrient loading from human land uses in the surrounding watersheds. One consequence of increased nutrient levels is proliferation of opportunistic macroalgae. We sought to understand spatial and temporal dynamics of ephemeral macroalgal mats and to examine their effects on salt marsh in a eutrophic estuary in central California. A time series analysis spanning 80years revealed that algal wrack has increased exponentially in frequency on the salt marsh, and was highly correlated with nutrient concentrations in the estuary, which have increased along with fertilizer use. Analysis of sediment δ15N showed a dramatic increase in nutrient loads attributable to agricultural fertilizer over the past 50years. We monitored 15 salt marsh plots along the bank edge and detected a negative relationship between algal wrack cover and salt marsh cover, flowering, and canopy height. Moreover, algal wrack led to retreat of vegetation from the bank edge, and increased bank erosion. We also experimentally added algal wrack to salt marsh edge plots. Algal addition decreased salt marsh cover, flowering, and canopy height, and increased retreat rate. By integrating time series analyses, isotope data, algal and marsh monitoring and manipulative experiments, we have identified robust linkages between increased anthropogenic nutrient loading, increased algal wrack cover, reduction in marsh resilience and conversion of marsh habitat to mudflat through bank erosion. Decreasing nutrient inputs to eutrophic estuaries is thus essential for conservation and restoration of salt marshes and enhancing their resilience in the face of sea level rise.

Environmental stress mediates trophic cascade strength and resistance to invasion

AbstractAlthough much attention has focused on the drivers of trophic cascade strength among systems (e.g., habitat type, body size, predator, and prey thermy) comparatively little is known of how trophic cascades change within systems, especially with respect to gradients in environmental stress. To address this, we measured the strength of a trophic cascade across an estuarine stress gradient and tested the predictions of “consumer stress model” that suggests sensitivity to stress is greater in mobile consumers relative to sessile prey. We used field observations and experimental manipulations within a central California estuary (Tomales Bay), where higher water temperatures in summer/fall and lower salinity during winter/spring may differentially affect predators (native crabs), meso‐consumers (invasive oyster drills), and sessile prey (native oysters). Our results demonstrate that sites with high environmental stress (warmer and lower salinity waters) excluded top predators from the food web, which facilitated high densities of oyster drills and resulted in low oyster survival. In contrast, less stressful sites (cooler and higher salinity waters) had high crab densities leading to reduced oyster drill survival and high oyster survival (an indirect positive effect). Observational data from 14 field sites along both margins of Tomales Bay are consistent with the results of experimental manipulations. This study highlights the importance of environmental stress in mediating within system trophic cascade strength by altering top predator abundance. Our results support the predictions of the consumer stress model and suggest that increasing environmental stress (warming and variable salinity regimes) as a result of climate change may decouple species interactions. Furthermore, warming and extreme low salinities may indirectly intensify the negative effects of non‐native species by reducing the biotic resistance provided by native predators.

Overgrowth of eelgrass by the invasive colonial tunicate Didemnum vexillum: Consequences for tunicate and eelgrass growth and epifauna abundance

The invasive tunicate Didemnum vexillum has become widely distributed in estuaries in western North America over the past decade. It has recently invaded eelgrass meadows in a central California estuary, growing directly on shoots and rhizomes. This invasion is of concern due to the important role of eelgrass as a foundation species in coastal communities and the known detrimental effects of this tunicate on sessile organisms in other habitats. The effects of tunicate overgrowth on eelgrass growth and morphology were studied using surveys, field experiments, and outdoor mesocosm experiments. In field and mesocosm experiments, there were decreases in eelgrass aboveground growth relative to unfouled shoots. Comparison of D. vexillum growth rates on eelgrass vs. hard substrate in mesocosms showed that growth was higher on hard substrate. The effects of D. vexillum overgrowth on the eelgrass epifaunal community were also characterized; overall invertebrate densities, and specifically tanaid and polychaete densities, were higher on shoots fouled with D. vexillum. Overall, the mean cover of D. vexillum on eelgrass shoots in the field was approximately 15–20% of shoot length. As the eelgrass in the mesocosms had less D. vexillum coverage than in the field, these results are conservative and suggest that relatively low D. vexillum coverage of individual eelgrass shoots may result in modest impacts on eelgrass.

Climate mediates hypoxic stress on fish diversity and nursery function at the land–sea interface

Coastal ecosystems provide numerous important ecological services, including maintenance of biodiversity and nursery grounds for many fish species of ecological and economic importance. However, human population growth has led to increased pollution, ocean warming, hypoxia, and habitat alteration that threaten ecosystem services. In this study, we used long-term datasets of fish abundance, water quality, and climatic factors to assess the threat of hypoxia and the regulating effects of climate on fish diversity and nursery conditions in Elkhorn Slough, a highly eutrophic estuary in central California (United States), which also serves as a biodiversity hot spot and critical nursery grounds for offshore fisheries in a broader region. We found that hypoxic conditions had strong negative effects on extent of suitable fish habitat, fish species richness, and abundance of the two most common flatfish species, English sole (Parophrys vetulus) and speckled sanddab (Citharichthys stigmaeus). The estuary serves as an important nursery ground for English sole, making this species vulnerable to anthropogenic threats. We determined that estuarine hypoxia was associated with significant declines in English sole nursery habitat, with cascading effects on recruitment to the offshore adult population and fishery, indicating that human land use activities can indirectly affect offshore fisheries. Estuarine hypoxic conditions varied spatially and temporally and were alleviated by strengthening of El Niño conditions through indirect pathways, a consistent result in most estuaries across the northeast Pacific. These results demonstrate that changes to coastal land use and climate can fundamentally alter the diversity and functioning of coastal nurseries and their adjacent ocean ecosystems.

Fortnightly Tidal Modulations Affect Net Community Production in a Mesotidal Estuary

Optical in situ chemical sensors enable sampling intervals and durations that rival acoustic techniques used for measuring currents. Coupling these high-frequency biogeochemical and physical measurements in estuaries to address ecosystem-scale questions, however, is still comparatively novel. This study investigated how tides affect ecosystem metabolism in a mesotidal estuary in central California (Elkhorn Slough). Dissolved oxygen measurements were used to estimate the terms in a control volume budget for a tidal creek/marsh complex at tidal timescales over several weeks. Respiration rates were 1.6 to 7.3 g O2 m−2 day−1; net community production approached 20 g O2 m−2 day−1. We found that aquatic NCP integrated throughout the creek complex varied significantly over the spring-neap cycle. The intertidal contribution to aquatic metabolism was net heterotrophic during spring tides and generally in balance during neap tides because spring-tide marsh inundation was limited to nighttime, and therefore the marsh could not contribute any primary production to the water column. At the estuary scale, the fortnightly export of oxygen from the main channel to the intertidal was largely balanced by an advective flux up-estuary.

Impacts of pesticides in a Central California estuary

Recent and past studies have documented the prevalence of pyrethroid and organophosphate pesticides in urban and agricultural watersheds in California. While toxic concentrations of these pesticides have been found in freshwater systems, there has been little research into their impacts in marine receiving waters. Our study investigated pesticide impacts in the Santa Maria River estuary, which provides critical habitat to numerous aquatic, terrestrial, and avian species on the central California coast. Runoff from irrigated agriculture constitutes a significant portion of Santa Maria River flow during most of the year, and a number of studies have documented pesticide occurrence and biological impacts in this watershed. Our study extended into the Santa Maria watershed coastal zone and measured pesticide concentrations throughout the estuary, including the water column and sediments. Biological effects were measured at the organism and community levels. Results of this study suggest the Santa Maria River estuary is impacted by current-use pesticides. The majority of water samples were highly toxic to invertebrates (Ceriodaphnia dubia and Hyalella azteca), and chemistry evidence suggests toxicity was associated with the organophosphate pesticide chlorpyrifos, pyrethroid pesticides, or mixtures of both classes of pesticides. A high percentage of sediment samples were also toxic in this estuary, and sediment toxicity occurred when mixtures of chlorpyrifos and pyrethroid pesticides exceeded established toxicity thresholds. Based on a Relative Benthic Index, Santa Maria estuary stations where benthic macroinvertebrate communities were assessed were degraded. Impacts in the Santa Maria River estuary were likely due to the proximity of this system to Orcutt Creek, the tributary which accounts for most of the flow to the lower Santa Maria River. Water and sediment samples from Orcutt Creek were highly toxic to invertebrates due to mixtures of the same pesticides measured in the estuary. This study suggests that the same pyrethroid and organophosphate pesticides that have been shown to cause water and sediment toxicity in urban and agriculture water bodies throughout California, have the potential to affect estuarine habitats. The results establish baseline data in the Santa Maria River estuary to allow evaluation of ecosystem improvement as management initiatives to reduce pesticide runoff are implemented in this watershed.

Environmental fate of fungicides and other current-use pesticides in a central California estuary

The current study documents the fate of current-use pesticides in an agriculturally-dominated central California coastal estuary by focusing on the occurrence in water, sediment and tissue of resident aquatic organisms. Three fungicides (azoxystrobin, boscalid, and pyraclostrobin), one herbicide (propyzamide) and two organophosphate insecticides (chlorpyrifos and diazinon) were detected frequently. Dissolved pesticide concentrations in the estuary corresponded to the timing of application while bed sediment pesticide concentrations correlated with the distance from potential sources. Fungicides and insecticides were detected frequently in fish and invertebrates collected near the mouth of the estuary and the contaminant profiles differed from the sediment and water collected. This is the first study to document the occurrence of many current-use pesticides, including fungicides, in tissue. Limited information is available on the uptake, accumulation and effects of current-use pesticides on non-target organisms. Additional data are needed to understand the impacts of pesticides, especially in small agriculturally-dominated estuaries.

Decreased solar radiation and increased temperature combine to facilitate fouling by marine non-indigenous species

Studies of the effects of climate changes on marine biofouling have mainly focused on the effects of temperature increase, but a decrease in the level of solar radiation could also influence the establishment and persistence of fouling species. To test if decreased solar radiation and/or increased temperature influenced marine fouling communities, solar radiation, and temperature were manipulated by deploying shading devices in the intertidal zone of a central California estuary. Non-indigenous species (NIS) recruiting to artificial substrata had greater coverage under the shading treatments than under transparent plates, indicating that low radiation facilitates recruitment and growth of NIS. In contrast, the coverage of NIS underneath warmer black plates was higher than that on white plates. Furthermore, spatial comparisons of recruitment showed that NIS had a tendency to grow better in the warmer region of the estuary whereas native species showed the opposing trend. The results suggest that both lower radiation and higher temperature may facilitate the spread of marine NIS.

Contrasting Seasonal and Fortnightly Variations in the Circulation of a Seasonally Inverse Estuary, Elkhorn Slough, California

Three and a half years of hydrographic, velocity, and meteorological observations are used to examine the dynamics of upper Elkhorn Slough, a seasonally inverse, shallow, mesotidal estuary in central California. The long-term observations revealed that residual circulation in Elkhorn Slough is seasonally variable, with classic estuarine circulation in the winter and inverse estuarine circulation in the summer. The strength of this exchange flow varied both within years and between years, driven by the annual cycle of dry summers and wet winters. Subtidal circulation is a combination of both tidal and density-driven mechanisms. The subtidal magnitude and reversal of the exchange flows is controlled primarily by the density gradient despite the significant tidal energy. As the density gradient weakens, the underlying tidal processes generate vertically sheared exchange flows with the same sign as that expected for an inverse density gradient. The inverse density gradient may then further strengthen this inverse circulation. These data were collected as part of the Land/Ocean Biogeochemical Observatory and demonstrate the utility of long-term in situ measurements in a coastal system, as consideration of such a wide range of forcing conditions would not have been possible with a less comprehensive data set.

Salt Marsh as a Coastal Filter for the Oceans: Changes in Function with Experimental Increases in Nitrogen Loading and Sea-Level Rise

Coastal salt marshes are among Earth's most productive ecosystems and provide a number of ecosystem services, including interception of watershed-derived nitrogen (N) before it reaches nearshore oceans. Nitrogen pollution and climate change are two dominant drivers of global-change impacts on ecosystems, yet their interacting effects at the land-sea interface are poorly understood. We addressed how sea-level rise and anthropogenic N additions affect the salt marsh ecosystem process of nitrogen uptake using a field-based, manipulative experiment. We crossed simulated sea-level change and ammonium-nitrate (NH4NO3)-addition treatments in a fully factorial design to examine their potentially interacting effects on emergent marsh plants in a central California estuary. We measured above- and belowground biomass and tissue nutrient concentrations seasonally and found that N-addition had a significant, positive effect on a) aboveground biomass, b) plant tissue N concentrations, c) N stock sequestered in plants, and d) shoot:root ratios in summer. Relative sea-level rise did not significantly affect biomass, with the exception of the most extreme sea-level-rise simulation, in which all plants died by the summer of the second year. Although there was a strong response to N-addition treatments, salt marsh responses varied by season. Our results suggest that in our site at Coyote Marsh, Elkhorn Slough, coastal salt marsh plants serve as a robust N trap and coastal filter; this function is not saturated by high background annual N inputs from upstream agriculture. However, if the marsh is drowned by rising seas, as in our most extreme sea-level rise treatment, marsh plants will no longer provide the ecosystem service of buffering the coastal ocean from eutrophication.

Experimental Test of the Effects of a Non‐Native Invasive Species on a Wintering Shorebird

The abundance of nearly one-quarter of the world's shorebird species is declining. At the same time, the number of non-native species in coastal ecosystems is increasing rapidly. In some cases, non-native species may affect negatively the abundance and diversity of shorebird prey species. We conducted an experimental study of the effects of the introduced European green crab (Carcinus maenas) on prey consumption by wintering Dunlin (Calidris alpina) in a central California estuary. We placed green crabs and Dunlin sequentially in field enclosures and measured changes in density of benthic invertebrate prey (e.g. polychaetes and small clams), Dunlin biomass, and gut contents of both Dunlin and crabs and observed foraging behavior of Dunlin. Green crabs significantly affected Dunlin foraging success through both direct and indirect multitrophic linkages. In enclosures with high densities of green crabs, crab foraging reduced the availability of polychaetes, and Dunlin consumed significantly fewer polychaetes compared with Dunlin in enclosures without crabs. High densities of green crabs were also associated with increased availability of small clams. Dunlin consumed significantly more small clams compared with Dunlin in enclosures without crabs. In our literature survey of studies of effects of non-native invasive species on shorebirds, we found three prior experiments that addressed the effect of non-native invasive species on shorebirds. Results of two of these studies showed positive direct effects of non-native invertebrates on shorebirds, 1 showed negative direct effects of a non-native plant on shorebirds through habitat conversion, and none showed indirect effects of non-native invertebrates. We suggest future management of shorebirds explicitly examine how non-native marine species, particularly invertebrates, directly and indirectly affect shorebirds.

Identifying factors that influence expression of eutrophication in a central California estuary

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 439:31-43 (2011) - DOI: https://doi.org/10.3354/meps09295 Identifying factors that influence expression of eutrophication in a central California estuary Brent B. Hughes1,2,*, John C. Haskins1, Kerstin Wasson1,2, Elizabeth Watson1,3 1Elkhorn Slough National Estuarine Research Reserve, Watsonville, California 95076, USA 2Department of Ecology and Evolutionary Biology, University of California Santa Cruz, Santa Cruz, California 95064, USA 3ORD-NHEERL, Atlantic Ecology Division, US Environmental Protection Agency, Narragansett, Rhode Island 02882, USA *Email: bbhughes@ucsc.edu ABSTRACT: Coastal eutrophication models have proposed that various environmental conditions can serve as filters mediating the effects of nutrient loading on coastal ecosystems. Variation in such filters due to natural or anthropogenic causes can potentially lead to varied responses in overall eutrophication expression as well as in individual eutrophication indicators. In this study, we sought to identify factors that affect eutrophication expression at contrasting sites within one nutrient-loaded estuary in central California. We developed and applied a eutrophication expression index to 18 sites in the Elkhorn Slough estuary and then used principal components analysis of environmental drivers (nutrients) and filters to determine how they relate to overall eutrophication expression as well as to individual eutrophication indicators. We also examined the relationship between one key filter, tidal range, and eutrophication indicators. Elkhorn Slough was determined to be a moderately eutrophic estuary, with individual sites varying from being low to hypereutrophic. Eutrophication expression was explained mostly by tidal range, depth, temperature, salinity, distance to estuary mouth, and turbidity, but not by nutrient concentrations. Tidal range in particular correlated strongly with most eutrophication indicators. Sites with artificially dampened tidal range through water control structures were more eutrophic than sites with full tidal exchange. Results from this study emphasize the importance of filters in mediating the negative ecological effects of eutrophication. Coastal managers can decrease eutrophication expression at a local scale by managing for filters (e.g. increasing tidal exchange to managed wetlands), complementing efforts to reduce eutrophication at a regional scale by decreasing nutrient loading. KEY WORDS: Eutrophication · Elkhorn Slough · Hypoxia · Environmental filters · Tidal range · Ulva · Chl a Full text in pdf format Supplementary material PreviousNextCite this article as: Hughes BB, Haskins JC, Wasson K, Watson E (2011) Identifying factors that influence expression of eutrophication in a central California estuary. Mar Ecol Prog Ser 439:31-43. https://doi.org/10.3354/meps09295 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 439. Online publication date: October 20, 2011 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2011 Inter-Research.