Gregory D. Williams

Californian Salt-Marsh Vegetation: An Improved Model of Spatial Pattern

ABSTRACT Although tidal wetland vegetation patterns are typically related to elevation, we hypothesized that the vertical range of a species may shift where the topography is more heterogeneous. We examined plant species occurrences in relation to elevation, proximity to the bay, and proximity to tidal creeks at a near-pristine wetland in San Quintín Bay, Baja California, Mexico. At the whole-wetland scale, most species occurred primarily within a 30-cm elevation band (the marsh plain). However, Spartina foliosa occurred only at the bayward margin, even though “suitable” elevations were present further inland. A similar pattern was found in San Diego Bay. At the microtopographic scale, three species on the marsh plain were strongly influenced by elevation, whereas four species responded to both elevation and proximity to tidal creeks. The latter species tended to “avoid” the lower 10 cm of the marsh plain except near a tidal creek. Species richness was thus greater (by 0.6 species at the lowest 10-cm class) at the tidal creek margin. Better drainage near creeks is the hypothesized cause. Our results help explain why species that are transplanted to constructed wetlands do not always grow at the full range of elevations they occupy in natural wetlands. We recommend that species be introduced to their modal elevation (determined from nearby reference marshes) and that salt-marsh construction designs include topographic heterogeneity (complex tidal creek networks). The analysis of broad-scale and fine-scale patterns of occurrence also suggests new habitat nomenclature. Elevation-based terms (“low,”“middle,” and “high” marsh) should be replaced by a system that considers elevation, landscape position, and conspicuous species. We suggest three habitat designations: (a) the high marsh—a 30- to 70-cm elevation range with Salicornia subterminalis; (b) the marsh plain—a 30-cm elevation range with heterogeneous topography and up to nine common species; and (c) cordgrass habitat—the bayward portion of the marsh plain and lower elevations, all occupied by Spartina foliosa. Although these habitats do not have discrete boundaries, separate terms are needed for wetland restoration plans and these designations will improve recognition that vegetation patterns respond to horizontal, as well as vertical, position.

Fish Assemblage Composition in Constructed and Natural Tidal Marshes of San Diego Bay: Relative Influence of Channel Morphology and Restoration History

This study evaluated the use by fish of restored tidal wetlands and identified links between fish species composition and habitat characteristics. We compared the attributes of natural and constructed channel habitats in Sweetwater Marsh National Wildlife Refuge, San Diego Bay, California, by using fish monitoring data to explore the relationships between channel environmental characteristics and fish species composition. Fishes were sampled annually for 8 yr (1989–1996) at eight sampling sites, four in constructed marshes and four in natural marshes, using beach seines and blocking nets. We also measured channel habitat characteristics, including channel hydrology (stream order), width and maximum depth, bank slope, water quality (DO, temperature, salinity), and sediment composition. Fish colonization was rapid in constructed channels, and there was no obvious relationship between channel age and species richness or density. Total richness and total density did not differ significantly between constructed and natural channels, although California killifish (Fundulus parvipinnis) were found in significantly higher densities in constructed channels. Multivariate analyses showed fish assemblage composition was related to channel habitat characteristics, suggesting a channel’s physical properties were more important in determining fish use than its restoration status. This relationship highlights the importance of designing restoration projects with natural hydrologic features and choosing proper assessment criteria in order to avoid misleading interpretations of constructed channel success. We recommend that future projects be designed to mimic natural marsh hydrogeomorphology and diversity more closely, the assessment process utilize better estimates of fish habitat function (e.g., individual and community-based species trends, residence time, feeding, growth) and reference site choice, and experimental research be further incorporated into the restoration process.

Fish use of tidal creek habitats in two southern California salt marshes

Fish assemblages from small intertidal creeks (first-order) and from larger subtidal creeks (third- and fourth-order) were sampled seasonally for 1 year at Tijuana Estuary and Sweetwater Marsh in southern California, USA. First-order creeks were dominated by Gillichthys mirabilis, Fundulus parvipinnis, and Clevelandia ios at both sites. At Sweetwater Marsh, species composition and species richness differed between creek orders, as did size structure of F. parvipinnis, one of the most abundant marsh residents; a greater proportion of small juveniles was collected from first-order creeks. At Tijuana Estuary, the larger system, elevations of first-order creeks were higher and fish use of creeks was lower than at Sweetwater Marsh. Shallow habitats are used extensively by several marsh residents, and may provide nursery habitat for juvenile F. parvipinnis. Intertidal creeks should be considered in designs of marsh restoration projects where the creation of habitat for resident fishes is a priority.

Influence of physical processes on the design, functioning and evolution of restored tidal wetlands in California (USA)

The performance of two intertidal wetland mitigation projects constructed by the California Department of Transportation (Caltrans) in the Sweetwater Marsh National Wildlife Refuge (SMNWR) in San Diego Bay was evaluated over 5 years. Most of the Sweetwater wetland complex has been altered this century, including diking (with subsequent subsidence), filling, modification of the tidal regime, freshwater inflow and sediment fluxes. The mitigation project goals included a range of functional criteria intended to support two endangered bird species (light-footed clapper rail and California least tern) and one endangered plant (salt marsh birds-beak). While the mitigation projects have achieved some of the performance criteria established in the regulatory permits (particularly, those related to fish), vegetation criteria for one of the bird species have not been met. The initial grading (in relation to local tidal datums) should support the target plant species, but growth has been less than required. Shortcomings of the habitat include elevated soil and groundwater salinity, low nutrient levels (especially nitrogen, which is readily leached from the coarse substrate), and eroding topography (where a single oversized and overly sinous channel and the lower-than-natural marshpalin result in high velocity surface water flow and erosion). The failure to achieve a large plain at low-marsh elevations highlights the importance of a more complete understanding of the relationship between the site physical processes (topography, hydrology, climate, geomorphology), substrate conditions, and biotic responses.

The importance of marsh access to growth of the California killifish, Fundulus parvipinnis, evaluated through bioenergetics modeling.

Abstract We developed, tested, and applied a bioenergetics model for the California killifish, Fundulus parvipinnis , to estimate the benefits of vegetated salt marsh habitats to killifish growth. Three independent statistical tests indicated that the model accurately estimated killifish growth. Predicted growth differed on average by only 14% from observed values. Field estimates of food consumption show that killifish can potentially double their daily food intake by adding marsh surface foods to their diet during each 2–3 h period of marsh access. Our model predicts that killifish grow from 20 to 44% faster if they add intertidal marsh surfaces to their subtidal feeding areas, despite higher metabolic costs and lower food assimilation during marsh feeding. Killifish may potentially grow up to 100% faster if energy costs associated with marsh access are minimized. During March–May, daytime tides are not high enough to permit marsh access. At water temperatures above 20°C, growth of adult killifish (>3.5 g wet mass, >60–62 mm, TL) is more adversely affected than that of juveniles by lack of marsh access. Killifish that are spawned early (April) develop under thermal regimes and periods of marsh access that differ from those spawned late (September), leading to large differences in the mass that could be achieved by the next spawning season. At age 1 year, early-spawned (ES) fish are 35–50% larger than late-spawned fish. Bioenergetics model are useful for determining the value of vegetated areas for fish (an important mitigation issue), designing for coastal wetland restoration (e.g. inclusion of marshes), and maintaining salt marshes as important fish habitat in southern California.