Walter G. Nelson

Patterns in Estuarine Vegetation Communities in Two Regions of California: Insights from a Probabilistic Survey

Monitoring wetlands at the ecoregion level provides information beyond the site scale and can inform regional prioritization of management and restoration projects. Our study was a component of the United States Environmental Protection Agency’s 2002 Environmental Monitoring and Assessment Program Western Pilot and is the first quantitative comparison of regional condition of California estuarine wetland plant communities. We measured indicators of estuarine emergent wetland condition in southern California and San Francisco Bay at probabilistically selected sites. In southern California, we also assessed potential anthropogenic stressors (presence of modified tidal hydrology, intensity of surrounding land use, and population density). Southern California salt marsh exhibited higher species diversity and greater percent cover of invasives. Seven of eight common plant species showed less variation in their distributions (zonation) across the marsh in southern California than in San Francisco Bay. Modified tidal hydrology was associated with absence, in our data, of certain native species, and higher relative percent cover of invasives across the marsh; however, our measures of landscape-level anthropogenic stress did not correlate with cover of invasives. We discuss lessons learned regarding the use of probabilistic site selection combined with our spatially complex data-collection arrays, and comment on utility of our protocol and indicators.

Development of an epiphyte indicator of nutrient enrichment: Threshold values for seagrass epiphyte load

Metrics of epiphyte load on macrophytes were evaluated for use as quantitative biological indicators for nutrient impacts in estuarine waters, based on review and analysis of the literature on epiphytes and macrophytes, primarily seagrasses, but including some brackish and freshwater rooted macrophyte species. An approach is presented that empirically derives threshold epiphyte loads which are likely to cause specified levels of decrease in macrophyte response metrics such as biomass, shoot density, percent cover, production and growth. Data from 36 studies of 10 macrophyte species were pooled to derive relationships between epiphyte load and -25 and -50% seagrass response levels, which are proposed as the primary basis for establishment of critical threshold values. Given multiple sources of variability in the response data, threshold ranges based on the range of values falling between the median and the 75th quantiles of observations at a given seagrass response level are proposed rather than single, critical point values. Four epiphyte load threshold categories - low, moderate, high, very high, are proposed. Comparison of values of epiphyte loads associated with 25 and 50% reductions in light to macrophytes suggest that the threshold ranges are realistic both in terms of the principle mechanism of impact to macrophytes and in terms of the magnitude of resultant impacts expressed by the macrophytes. Some variability in response levels was observed among climate regions, and additional data collected with a standardized approach could help in the development of regionalized threshold ranges for the epiphyte load indicator.

Intertidal habitat utilization patterns of birds in a Northeast Pacific estuary

A habitat-based framework is a practical method for developing models (or, ecological production functions, EPFs) to describe the spatial distribution of ecosystem services. To generate EPFs for Yaquina estuary, Oregon, USA, we compared bird use patterns among intertidal habitats. Visual censuses were used to quantify abundance of bird groups and general species richness in: Zostera marina (eelgrass), Upogebia (mud shrimp)/mudflat, Neotrypaea (ghost shrimp)/sandflat, Zostera japonica (Japanese eelgrass), and low marsh estuarine habitats. Also assessed were (1) spatial variation within a habitat along the estuary gradient and, (2) temporal variation based on bi-monthly samples over a year at five tidal ranges. Z. marina was an important estuarine habitat based on nearly all metrics of bird use, except for shorebird densities. This suggests that reductions in native eelgrass habitat may reduce the abundance and diversity of birds in Yaquina estuary. Our results suggest that a habitat based assessment approach is generally feasible for developing relative EPFs related to the presence of birds within estuarine systems.

Development of an epiphyte indicator of nutrient enrichment: A critical evaluation of observational and experimental studies

An extensive review of the literature describing epiphytes on submerged aquatic vegetation (SAV), especially seagrasses, was conducted in order to evaluate the evidence for response of epiphyte metrics to increased nutrients. Evidence from field observational studies, together with laboratory and field mesocosm experiments, was assembled from the literature and evaluated for a hypothesized positive response to nutrient addition. There was general consistency in the results to confirm that elevated nutrients tended to increase the load of epiphytes on the surface of SAV, in the absence of other limiting factors. In spite of multiple sources of uncontrolled variation, positive relationships of epiphyte load to nutrient concentration or load (either nitrogen or phosphorus) often were observed along strong anthropogenic or natural nutrient gradients in coastal regions. Such response patterns may only be evident for parts of the year. Results from both mesocosm and field experiments also generally support the increase of epiphytes with increased nutrients, although outcomes from field experiments tended to be more variable. Relatively few studies with nutrient addition in mesocosms have been done with tropical or subtropical species, and more such controlled experiments would be helpful. Experimental duration influenced results, with more positive responses of epiphytes to nutrients at shorter durations in mesocosm experiments versus more positive responses at longer durations in field experiments. In the field, response of epiphyte biomass to nutrient additions was independent of climate zone. Mesograzer activity was a critical covariate for epiphyte response under experimental nutrient elevation, but the epiphyte response was highly dependent on factors such as grazer identity and density, as well as nutrient and ambient light levels. The balance of evidence suggests that epiphytes on SAV will be a useful indicator of persistent nutrient enhancement in many situations. Careful selection of appropriate temporal and spatial constraints for data collection, and concurrent evaluation of confounding factors will help increase the signal to noise ratio for this indicator.

Patterns of shading tolerance determined from experimental light reduction studies of seagrasses

A review and analysis of the experimental literature on seagrass shading evaluated the relationships among experimental light reduction, experimental duration, additional modifying factors and common meadow-scale seagrass response metrics to determine whether there were consistent statistical relationships. Modifying factors included study latitude, field site depth, season of experiment initiation, rhizome connectivity (severed, intact), experiment type (field, mesocosm), and seagrass life history strategy. Highly significant, best fit linear regression models were found for both biomass and shoot density reduction that included light reduction, duration and other modifying variables, although unexplained variation in the data were high. Duration of light limitation affected extent of response for both metrics, and unexplained variance was greatly reduced by analysis of data from durations >60d for shoot density and for >60d <120d for biomass. Life history strategy was also a significant factor in three of four regression models. While the slopes of the responses were relatively similar for biomass and shoot density, unexplained variation was generally greater for shoot density than biomass in models for data pooled across species. There were highly significant, best fit regression models found for both biomass and shoot density for both genus and species level analyses, with the extent and duration of light reduction the most important model factors. Season of experiment, rhizome status, latitude, and experiment type all were also included in multiple models. Biomass regression models again tended to have lesser unexplained variation than shoot density models. Life history was invariant within genus and species, and separate analyses for data divided among Colonizing, Opportunistic, and Persistent strategies found relatively similar, best fit regression models among strategies. However, the mean percent reduction of both biomass and shoot density was generally lower for the Persistent strategy than for the other two life histories, suggesting a greater buffering capacity against effects of light reduction for such species. Overall, biomass based models explained more of the variance in seagrass response to light reduction than shoot density, and may be the preferred response variable for meadow-scale impact assessments. The relationships observed may inform management decisions by helping define the scope of expected responses of seagrasses in general to the range of factors that may reduce light availability to seagrasses.

An evaluation of factors controlling the abundance of epiphytes on Zostera marina along an estuarine gradient in Yaquina Bay, Oregon, USA

Epiphytes on seagrass (Zostera marina) growing in the lower intertidal were examined along an estuarine gradient within Yaquina Bay, Oregon over a period of 4 years. The Yaquina Estuary receives high levels of nutrients from the watershed during the wet season and from the ocean during the dry season. Mean epiphyte biomass per unit seagrass leaf surface area (epiphyte load) peaked during the summer, and thus epiphyte load was higher during dry season than wet season in both marine and riverine dominated regions. Epiphyte load was greater in marine than in riverine dominated areas in both wet and dry seasons, although only dry season differences were significant. There was no evidence that grazers controlled epiphyte load differences. Annual DIN concentration was inversely related to epiphyte load, principally because of elevated wet season dissolved inorganic nitrogen from river inputs. While there was a positive annual relation of epiphyte load to PO4 concentration, it is not clear that phosphorus becomes a limiting nutrient for epiphyte growth. Water column light attenuation tends to increase linearly with distance from the estuary mouth, while both epiphyte load and Z. marina biomass tend to decrease. Both seagrass and seagrass epiphytes may be increasingly light limited in the upper estuary, and thus, epiphyte loads may have proportionally more impact on seagrass occurrence in this estuarine region.