Elkhorn Slough Research Articles

Biogeochemical processes in a small California estuary. 1. Benthic fluxes and pore water constituents reflect high nutrient freshwater inputs

Elkhorn Slough, a small estuary in central California, receives nutrient inputs from agri- cultural and other non-point source runoff. To evaluate the effect of nutrient loading on ecosystem processes, rates of sediment oxygen consumption, benthic nutrient fluxes, pore water ammonium (NH4 + ), dissolved inorganic phosphate (DIP) and sulfide (S 2- ) concentrations were measured at 5 sites several times each year between 1998 and 1999. Two sites near the head of the slough receive direct runoff from agricultural fields, while the sites in the middle reaches and mouth receive direct runoff from grasslands and woodlands. Sites receiving agricultural runoff usually had high water column concentrations of dissolved inorganic nitrogen and chlorophyll a (50 µM and 30 µg l -1 , respectively). High precipitation in the winter of 1998 caused low salinities in the slough at that time. In general, salinity and nutrient concentrations were inversely correlated. Sites receiving agricultural runoff had high pore water NH4 + , DIP and dissolved S 2- concentrations, particularly in the summer. Oxygen pen- etration into sediments was the greatest at the sites adjacent to grasslands and woodlands, and least at the sites receiving high nutrient runoff. Sediment oxygen consumption was 4 times greater at sites near agricultural fields than the site near the mouth, while NH4 + flux out of sediments was between 4 and 20 times greater at agricultural sites than at non-agricultural sites. This study suggests that high nutrient runoff has primarily a local rather than estuary-wide effect on sediment biogeochemical pro- cesses in Elkhorn Slough, perhaps due to longer water residence time at the head of the slough com- pared to the mouth.

Biological invasions of estuaries without international shipping: the importance of intraregional transport

Increased awareness of the problem of introduced marine species has led to recent surveys of several large bays with international shipping. To our knowledge, no thorough search for introductions has been carried out in an embayment not connected to an international harbor. In 1998, we investigated the macroinvertebrate fauna of Elkhorn Slough (ES), an estuary in central California. Fieldwork and a literature review revealed 56 known exotic species at ES, a surprising diversity considering the rather modest search effort, the relatively natural setting of this estuary, and the lack of international shipping. While some exotic species at ES were probably introduced directly from distant waters with cultivated oysters, others likely arrived more indirectly via San Francisco Bay or other regional ports with thriving populations of invaders, travelling for instance as adults fouling boats or as larvae on currents. The effect of international shipping, including ballast water dumping, is thus not limited to areas with major harbors, but rather reverberates up and down the coast to seemingly isolated embayments.

Vertical Distribution of Denitrification Potential, Denitrifying Bacteria, and Benzoate Utilization in Intertidal Microbial Mat Communities.

Aspects of denitrification and benzoate degradation were studied in two estuarine microbial mat communities on the California coast by measuring the depth distributions of potential denitrification rates, genetic potential for denitrification, nitrate concentration, benzoate mineralization rates, total bacterial abundance, and abundance of a denitrifying strain (TBD-8b) isolated from one of the sites. Potential denitrification was detected in microbial mat cores from both Elkhorn Slough and Tomales Bay. Maximum denitrification rates were more than two orders of magnitude higher at Elkhorn Slough (3.14 mmol N m -2 d -1) than at Tomales Bay (0.02 mmol N m -2 d -1), and at both sites, the maximum rates occurred in the 0-2 mm depth interval. Ambient pore [NO 3 + NO 2] was substantially higher at Elkhorn Slough than at Tomales Bay. Incorporation and mineralization of benzoate was maximal near the mat surface at Elkhorn Slough. The areal rate of benzoate utilization was 1045 nmol C m -2 d -1, which represented utilization of 70% of the added substrate in 24 h. Total bacterial and TBD-8b abundances were greatest near the surface at both Tomales Bay and Elkhorn Slough, and TBD-8b represented less than 0.2% of the total. Genetic potential for denitrification, quantified by hybridization with a nitrite reductase gene fragment, was present below the mat surface at average levels representing presence of the gene in approximately 10% of the total cells.

Case study of Elkhorn Slough Field, Solano County, and Grand Island Field, Sacramento County, California: Pre-3-D versus post-3-D field development

Elkhorn Slough Field in Solano County, California, is on the eastern flank of the Sacramento Valley, approximately 25 miles southwest of Sacramento. Grand Island Field is in Sacramento County, approximately five miles southeast of Elkhorn Slough Field (Figure 1). Both fields are between the updip edge of the Upper Winters sand pinchout and the beginning of the Winters structural play. Figure 1. Location map. The Winters pinchout play was defined with regional geologic mapping through the basin. It is turbiditic in nature, sands being transported through channels incised into the shelf and deposited into deepwater fans, surrounded by shales. The sands were deposited in massive pulses, one flow possibly cutting into and welding into the previous one. This makes it difficult to distinguish the sands from one another on the seismic because they thin in an updip position. The sands have excellent porosities (18–25%) and thicknesses up to 115 gross ft. In Elkhorn Slough and Grand Island Fields, the depths of these sands are approximately 8500–8600 ft. These elements have made the Winters in this area conducive to AVO analysis. In defining this play regionally, several key mapping tools were used to identify these prospects. They included structural noses due to differential compaction around the sand bodies (which create structure where the sands exist); net sand isopachs to delineate the existence of reservoir; and seismic amplitudes to show potential gas accumulation. In the Elkhorn Slough area, we began with 2-D seismic, mapped the structures, and used them with the regional sand maps to delineate possible fan limits. In 1992, in partnership with EOG Resources (formerly Enron Oil and Gas), Amerada Hess began drilling wells within the fan. The initial well, Enron's 22–1 Nixon (well X-1), was a gas producer from 15 ft of the A sand and had 42 ft of wet …

Modeling water, heat, and oxygen budgets in a tidally dominated estuarine pond

We present a methodology to model water, heat, and dissolved oxygen budgets on short time scales within a shallow estuarine environment using limited local water quality and climatic data. A tidally restricted eutrophic pond in Elkhorn Slough, California, experiences extreme diel dis- solved oxygen (DO) variations during warm sunny days and neap tidal cycles. Empirical relationships between biological metabolism (measured as DO variations) and physical parameters are derived using time-series hydrographic data from the site. A simple box model predicts the time-series struc- ture of water depth, water temperature, and DO concentrations within the pond as a function of the tidal cycle and local climate over 10 to 15 d simulation periods. Parametric analyses illustrate appli- cations of the model to predict thermal and geochemical responses to hydrologic, chemical, and cli- matic alterations to the system. This study also initiates preliminary investigations of the fundamen- tal processes influencing DO dynamics within a natural wetland environment, and could be used to explore the potential response to various physical and chemical alterations prior to restoration or management adjustments. Future box models of wetland geochemistry should include carbon and nutrient budgets to create a more sensitive model, and to allow more rigorous evaluations of the role decreased nutrient inputs would play in alleviating eutrophic conditions.

Overview of Shorebird Abundance and Distribution in Wetlands of the Pacific Coast of the Contiguous United States

We corrdinated censuses from April 1988 to April 1995 to obtain an overview of shorebird abundance and distribution in Pacific Coast wetlands of the contiguous United States. We attempted to acquire at least 3 years of data for all major wetlands from counts within a short time window each fall, winter, and spring. Fourteen abundant, 8 moderatcly abundant, and 21 rare-to-uncommon species were recorded. For temperate-zone breeders. peak periods of abundance were fall or winter and, when discernible for arctic breeders, mostly fall or spring. Arctic breeders were relatively more abundant than temperate-zone breeders in Oregon and Washington wetlands. All five of the most abundant temperate breeders were limited primarily to California wetlands in all seasons. Only for Western Sandpiper (Calidris mauri) and Dunlin (C. alpina) did the estimated total individuals present simultaneously in all wetlands exceed 100.000. Fifty-six of 66 sites surveyed had at least four counts and at least 100 shorebirds on one or more counts; 38 of the 56 sites held at least 1% of 1 of 13 key species during at least one season. San Francisco Bay accounted for 24-96% of the estimated totals for key species; Grays Harbor, Willapa Bay, Humboldt Bay, Tomales Bay, Point Reyes Esteros, Bolinas Lagoon, Elkhorn Slough, Morro Bay, Mugu Lagoon, Bolsa Chica, Mission Bay, and San Diego Bay held at least 1% of at least half the key species in at least one season. The usefulness of five criteria for selecting key wetlands for shorebird conservation are examined and potential threats are discussed.

Methane stable isotopic ratios and concentrations as indicators of methane dynamics in estuaries

Mixing diagrams of methane (CH4) concentration and stable isotopic ratio (δ13C‐CH4) were used to examine the fate of river‐borne CH4 as it crosses a variety of estuaries: Columbia River (Oregon/Washington), Parker River (Massachusetts), Great Bay (New Hampshire), Kaneohe Bay (Hawaii), and Elkhorn Slough (California). Unlike the surface of the open ocean, these systems are not in near atmospheric equilibrium with respect to concentration or δ13C‐CH4 value. The range of observed CH4 concentrations and δ13C‐CH4 values were 33–440 nM and −36 to −58 per mil, respectively, for the freshwater end‐members for these systems, 12–330 nM and −48 to −60 per mil for water at the mouths of the estuaries, and 1.6–6 nM and −45 to −60 per mil for the seawater end‐members. In the Kaneohe Bay estuary, CH4 concentration and δ13C‐CH4 displayed near‐conservative behavior. In the Columbia River estuary, there was loss of riverine CH4 coupled with shifts to heavier isotopic values, apparently the result of in situ CH4 oxidation; this oxidation exhibited an apparent kinetic isotopic fractionation factor of 1.0042−1.012. In contrast, the other estuaries showed elevated concentrations and more negative δ13C‐CH4 values apparently resulting from inputs of biogenic CH4 from midestuary marshes and sediments. The upper reaches of all these systems were well out of equilibrium with the atmosphere on a concentration basis, indicating that they are atmospheric CH4 sources. However, these first δ13C‐CH4 measurements show that there is a wide range of isotopic variation in these waters, which indicates that it will be difficult to estimate the collective isotopic contribution of estuaries to the global methane budget.

Trophic Ecology of the Dominant Fishes in Elkhorn Slough, California, 1974-1980

Food habits of the dominant fishes collected from 1974 to 1980 at eight locations in Elkhorn Slough, California, and the adjacent ocean were investigated. Epifaunal crustacea was the major prey group identified from stomach contents of more than 2,000 fishes, followed by epifaunal and infaunal worms, and molluscs. Overall, 18 fish species consumed 263 different prey taxa, ranging from 10 taxa to 125 taxa per fish species and including 99 crustacean, 56 polychaete, and 39 molluscan taxa. Mean prey richness was greatest at stations near the ocean and lowest at inshore stations. Detailed dietary data for all prey taxa were summarized as trophic spectra for each fish species. Trophic spectra represented functional groups of prey and were used for comparisons of dietary similarity. Cluster analyses, based on trophic spectra, resulted in four feeding guilds of fishes. Of 18 fish species, seven (Amphistichus argenteus, Leptocottus armatus, Embiotoca jacksoni, Clevelandia ios, Gillichthys mirabilis, Cymatogaster aggregata, andCitharichthys stimaeus) fed principally on epifaunal crustacea. Four species (Pleuronectes vetulus, Platichthys stellatus, Phanerodon furcatus, andMyliobatus californica) consumed mostly molluscs and infaunal worms. Two species (Psettichthys melanostictus andTriakis semifasciata) fed on mobile crustacea, and five species (Hyperprosopon anale, Engraulis mordax, Clupea pallasi, Atherinopsis californiensis, andAtherinops affinis) fed largely on zooplankton and plant material. Our results suggest that high food availability enhances the nursery function of imshore habitats, and emphasize the importance of invertebrate prey populations and the indirect linkage of plant production to the ichthyofaunal assemblarly marine immigrant species that are likely ‘estuarine dependent’.

Prehistoric Native American Fisheries of the Central California Coast

Over 77,000 fish remains from 51 archaeological sites on the central California coast between San Mateo and San Luis Obispo counties, deposited between 6200 B.c. and A.D. 1830, were studied to assess prehistoric species distribution, diversity, and Native American fisheries. Remains were obtained from exposed rocky coastal sites, lagoon–estuaries at Elkhorn Slough and Morro Bay, and the freshwater drainages of the Pajaro and Salinas rivers. On the rocky coast, 58.4% of the remains represented large inshore species, 26.9% were small schooling species, and 11.8% were surfperches (family Embiotocidae). Large inshore species included rockfishes Sebastes spp., lingcod Ophiodon elongates, kelp greenling Hexagrammos decagrammus, cabezon Scorpaenichthys marmoratus, and monkeyface prickleback Cebidichthys violaceus. At Elkhorn Slough and Morro Bay, about half of the remains of marine species represented moderately small schooling species, including Pacific herring Clupea pallasi, Pacific sardine Sardinops sagax, northern anchovy Engraulis mordax, topsmelt Atherinops affnis, jacksmelt Atherinopsis californiensis, and California grunion Leuresthes tenuis. Surfperches also were common, and specialized local fisheries for flounders or sharks and rays were suggested. Aquatic conditions at Elkhorn Slough were dramatically different from those that exist today. Sites on Elkhorn Slough had both marine and freshwater fishes and showed site occupation when the Salinas River entered the slough and did not follow its present course into Monterey Bay. Sacramento perch Archoplites interruptus was the most abundant species found at freshwater sites, and remains of extinct thicktail chub Gila crassicauda confirm its presence in the Pajaro and Salinas rivers. Surprisingly rare are the remains of steelhead (the anadromous form of rainbow trout) Oncorhynchus mykiss. As is the case today, Pacific salmon were apparently absent from central coast streams south of the San Lorenzo River. Presumably the remains reflect local species availability.

Assessment of genetic diversity of seagrass populations using DNA fingerprinting: implications for population stability and management.

Populations of the temperate seagrass, Zostera marina L. (eelgrass), often exist as discontinuous beds in estuaries, harbors, and bays where they can reproduce sexually or vegetatively through clonal propagation. We examined the genetic structure of three geographically and morphologically distinct populations from central California (Elkhorn Slough, Tomales Bay, and Del Monte Beach), using multilocus restriction fragment length polymorphisms (DNA fingerprints). Within-population genetic similarity (Sw) values for the three eelgrass populations ranged from 0.44 to 0.68. The Tomales Bay population located in an undisturbed, littoral site possessed a within-population genetic similarity (Sw = 0.44) that was significantly lower than those of the other two populations. Cluster analysis identified genetic substructure in only the undisturbed subtidal population (Del Monte Beach). Between-population similarity values (Sb) for all pairwise comparisons ranged from 0.47 to 0.51. The three eelgrass populations show significantly less between locale genetic similarity than found within populations, indicating that gene flow is restricted between locales even though two of the populations are separated by only 30 km. The study demonstrates that (i) natural populations of Z. marina from both disturbed and undisturbed habitats possess high genetic diversity and are not primarily clonal, (ii) gene flow is restricted even between populations in close proximity, (iii) an intertidal population from a highly disturbed habital shows much lower genetic diversity than an intertidal population from an undisturbed site, and (iv) DNA fingerprinting techniques can be exploited to understand gene flow and population genetic structure in Z. marina, a widespread and ecologically important species, and as such are relevant to the management of this coastal resource.

Mixed-function oxidase-specific activity in wild and caged speckled sanddabs Citharichthys stigmaeus in Elkhorn Slough, Moss Landing Harbor and nearshore Monterey Bay, California.

The goal of this study was to characterize the spatial, seasonal and annual hepatic activities of mixed-function oxidase (MFO) in the speckled sanddab Citharichthys stigmaeus, the most common fish in the Moss Landing area. In addition, techniques to monitor MFO activities in caged speckled sanddabs were developed and tested. Once the relationship between MFO activities in caged and wild fish populations is determined, caged fish could be used to monitor potential hydrocarbon impacts at Moss Landing, or other marine sites. During each of the spatial, seasonal and annual sediment samplings conducted in 1985-1987 as part of a separate hydrocarbon variability study at Moss Landing, 12 wild speckled sanddabs were collected from Moss Landing Harbor, Elkhorn Slough and nearshore Monterey Bay sites. In addition, four locations were chosen for a 14-day field caged fish experiment. The caged fish experiments successfully demonstrated the feasibility of using caged sanddabs as indicators of hydrocarbon exposure. The major source of variability in hepatic aryl hydrocarbon hydroxylase activity in wild speckled sanddabs from the Moss Landing area is due to seasonal rather than site differences. Significant relationships between caged fish MFO response and sediment hydrocarbon concentrations were found. It is possible that caged fish could be used in place of costly sediment sampling and analysis, and provide a more direct method to assess biological impacts. Many of the caging techniques demonstrated in this study can easily be transferred to other benthic flatfish, and other marine and freshwater environments.

Seasonal and annual distribution of organic contaminants in marine sediments from Elkhorn slough, moss landing harbor and nearshore Monterey Bay, California

This 3-year study provides data on the spatial, seasonal and annual variability of hydrocarbons and total organic carbon present in marine sediments at three sites: Elkhorn Slough, Moss Landing Harbor and nearshore Monterey Bay in the vicinity of Moss Landing, California. The study provides baseline information that could be used to evaluate the potential impacts of future fuel oil releases occuring in the Moss Landing area. Groups of hydrocarbons were chosen to represent the hydrocarbon inputs into the Moss Landing area. These included the pesticide dichlorodiphenyltrichloroethane (DDT), polychlorinated biphenyl (PCB), phthalic acid ester (PAE), polycyclic aromatic hydrocarbon (PAH) and combustion PAHs (ΣCOMBs). For ΣDDTs, ΣPCBs, ΣPAEs, ΣPAHs and ΣCOMBs, the major sources of variability were between sites and random effects. Subsites within each site contributed little variability. No significant seasonal differences in any chemical contaminant group were found at any site. Significant seasonal differences in total organic carbon (TOC) and significant annual differences in ΣPCBs, ΣPAHs, ΣCOMBs and ΣPAEs were found at the nearshore Monterey Bay site. Significant annual differences in ΣPAEs and TOC were found within Moss Landing Harbor, and significant annual differences in ΣPAEs were found within the Elkhorn Slough site. Implications for future sampling designs in the Moss Landing area are that given the current baseline conditions (a stable, low rate of hydrocarbon input), a variability of 75–150 m 2 may not need to be heavily sampled. Spatial variability, not seasonal or annual variability, is the major source of hydrocarbon variability in Moss Landing sediments, although 3 years may not be long enough to establish long-term annual trends. Further research to determine the ΣPAH spatial sampling scale for oil spills is needed.

Burrow environment and coelomic fluid characteristics of the echiuran worm Urechis caupo from populations at three sites in northern California

The burrow microhabitat and physiology of the echiuran worm Urechis caupo at a high-density site (Elkhorn Slough, California, USA), were investigated from 1987 to 1990 to determine physical and chemical conditions, worm density and distribution, and coelomic fluid characteristics such as heme composition, pO2, pH, and coelomocyte volume. During tidal exposure, worm burrows at this site exhibited, on average, 52% air-saturated water, 11μM burrow-water sulfide, 85 μg/mg wet weight sediment sulfide, and salinity and osmolalities similar to those of seawater. These conditions are compared to those of another California site, Bodega Bay, which had slightly lower oxygen concentrations, but higher water and sediment-sulfide levels. A more limited comparison to a third site, Princeton Harbor, California, is included. Worms from Bodega Bay, the higher sulfide site, had greater concentrations of hematin, a non-globin heme compound contained in the coelomocytes, and exhibited a greater tolerance to sulfide in the laboratory. These data are consistent with the hypothesis that hematin is a sulfide-detoxifying agent that may enhance survival of U. caupo in the sulfide-rich mudflat environment.

Characterization of disjunct populations of Zostera marina (eelgrass) from California: genetic differences resolved by restriction-fragment length polymorphisms

Comparative restriction-fragment analysis was used to analyze the nuclear ribosomal DNA, and alcohol dehydrogenase-1 loci of Zostera marina L., for variation within and among populations. Eelgrass is a perennial marine flowering plant that is widespread and ecologically significant throughout the temperate northern hemisphere. A chemical method was developed to obtain restriction-quality DNA without CsCl fractionation from experimentally relevant quantities of seagrass tissues (0.5 to 1.0 g). The yield was 25 μg g-1 fresh weight. The three morphologically distinct forms of Z. marina from disjunct populations examined in this study were found to be genetically distinct; morphologically similar populations were indistinguishable genetically. Genetic distinction also correlated with habitat depth, as subtidal and intertidal populations were clearly divergent. Homologous probes for the 17S and 28S ribosomal DNA genes were used to map 24 restriction sites on the rDNA repeat of Z. marina, which was determined to be about 14 kb in length. At least 1 length mutation and 5 restriction-site changes were identified that distinguished Z. marina populations from San Diego and Monterey Bay (Del Monte Beach) from Z. marina populations from Elkhorn Slough and Tomales Bay. Estimated sequence variation (100×p) between eelgrass populations ranged from 0.00 to 0.69. Individual plants were observed to contain as many as four different rDNA-repeat length variants. The mean number of rDNA-repeat length variants per individual in Z. marina was about two. Intrapopulation variation in rDNA-repeat type was observed in only one individual from the Tomales Bay population.

Seasonal and spatial patterns of ichthyoplankton abundance in Elkhorn Slough, California

Seasonal and spatial patterns in abundance of fish eggs and larvae are described from 323 samples collected from September 1974–September 1976 in Elkhorn Slough, a shallow, tidal embayment in Monterey Bay, California. These included 3645 larvae of 29 taxa that were numerically co-dominated by gobiid and clupeoid larvae. Ninety-four percent of the total catch comprised seven species, and included, in rank order abundance, Gillichthys mirabilis, Engraulis mordax, Clevelandia ios, Hypomesus pretiosus -osmerid, Leptocottus armatus, Genyonemus lineatus , and Clupea pallasii . Eggs of E. mordax, Citharichthys spp., and G. lineatus accounted for 73% of the catch. Two seasonal groups of larvae were evident. E. mordax and the gobiids ( G. mirabilis and C. ios ) formed a summer-fall group. While more speciose, a winter and early-spring group comprised of larval L. armatus, H. pretiosus -osmerid, Atherinidae and Ammodytes hexapterus was not as abundant as the summer–fall assemblage. Egg densities were overwhelmingly high in summer, due almost entirely to E. mordax . Similarity in species composition was greatest between collections from the most inland stations; larval assemblages from near-ocean stations were least similar to the inland slough assemblages. These distributions are attributed to reproductive specializations (egg type and spawning origin of adults) and hydrographic conditions.

Temporal and Spatial Patterns in Abundance and Diversity of Fish Assemblages in Elkhorn Slough, California

Assemblages of ichthyofauna of shallow inshore habitats along California’s central coast are described in terms of species composition, abundance, and life-style categories. A total of 22,334 fishes from 65 species and 27 families was collected with otter trawls at six sites in the main channel and tidal creeks of Elkhorn Slough, a tidal embayment and seasonal estuary, and two nearshore ocean stations in Monterey Bay during 44 months between August 1974 and June 1980. Greater than 90% of the catch comprised 10 species. The four dominant species,Cymatogaster aggregata, Leptocottus armatus, Phanerodon furcatus, andEmbiotoca jacksoni, occurred during most or all seasons and were classified as residents or partial residents. Several abundant species were marine immigrants that seasonally use the slough as spawning and nursery grounds; this resulted in higher abundance and species richness during summer. Species collected during winter largely were slough residents. Species compsosition and richness varied with distance from the slough entrance. The ocean assemblage was most different, and its similarity to other stations decreased progressively with distance inland and into the tidal creeks. During our study, 5,074 fishes were collected by beach seine in Bennett Slough, a remote shallow marsh basin adjacent to the entrance of Elkhorn Slough. Species richness was relatively low and three euryhaline species accounted for >80% of the total catch. The species assemblage was most similar to those at the tidal creek and most shallow stations of Elkhorn Slough. Resident species numerically dominated assemblages in Bennett Slough and the most inland areas of Elkhorn Slough. The high relative abundance of marine-related fishes (classified as marine, marine immigrant, and partial resident), entering Elkhorn Slough early in life or as spawning adults indicates the importance of this habitat to nearshore fish assemblages.

SEASONAL OCCURRENCE AND ABUNDANCE OF BRACHYURAN LARVAE IN A COASTAL EMBAYMENT OF CENTRAL CALIFORNIA

ABSTRACT Little information is available on the larval ecology of brachyuran crabs on the western coast of North America. Horizontal plankton tows were conducted biweekly over a 14-month period at 4 stations in Elkhorn Slough, California, and at 1 station 1 km offshore and adjacent to the slough. The brachyuran larvae of 5 families represented by 12 taxa were identified to the lowest possible taxon. Larvae of Pinnixa franciscana, Hemigrapsus oregonensis, Cancer gracilis, Pinnixaweymouthi, and Pachygrapsus crassipes occurred most frequently. Larvae were most abundant at all sites in early spring and late winter. One exception was the larvae of P. crassipes which had a peak abundance in late summer and early fall. Larvae of P. franciscana and P. weymouthi generally remained within Elkhorn Slough, whereas larvae of H. oregonensis and P. crassipes appeared to be transported offshore. The numerous first stage zoeae of P. Fanciscana. H. oregonensis, and P. crassipes collected in the upper Elkhorn Slough suggest that adult females use this habitat as a spawning ground.

Aspects of the Reproduction of the Bat Ray, Myliobatis californica, in Central California

Reproduction in bat rays collected from Elkhorn Slough in central California appears to follow a well-defined annual cycle in which mature individuals use the Slough in the spring and summer months to give birth and mate. The bat ray's gestation period is estimated to be between 9-12 mo. Embryo number in pregnant females ranged between 2-5. Disc width at birth ranged between 220305 mm. In male bat rays, three indicators of sexual maturity (presence of mature spermatozoa, clasper/disc-width relationship, and internal morphology) showed that at 2-3 yr of age sexual maturity occurs at a disc width of about 450-622 mm. In females, presence of mature ova, ranging from means of 25-279 per female, indicates that 50% maturity occurs at about 881 mm disc width and approx. 5 yr of age.

Sea otter foraging on deep-burrowing bivalves in a California coastal lagoon

Sea otter, Enhydra lutris, predation had no detectable effect on abundance and size distribution of deep-burrowing bivalve prey in the Elkhorn Slough, California, USA. Up to 23 otters were present for 6 mo of the study period (March 1984 through April 1985). This is in contrast to previous studies of sea otter predation, especially on the shallow-burrowing Pismo clam Tivela stultorum, which can be found along the wave-exposed coast near the slough. The deep-burrowing clams Tresus nuttallii and Saxidomus nuttalli made up 61% of the prey taken in the slough, and are more difficult for otters to excavate than Pismo clams. The occurrence of foraging otters was highest in an area where the two bivalve prey were extremely abundant (∼18 individuals m−2). However, the otters did not selectively prey on the largest clams available within the study sight, but foraged preferentially in a patch of smaller individuals where bivalve burrow depth was restricted by the presence of a dense clay layer. This foraging strategy maximized the amount of prey biomass obtained per unit volume of sediment excavated. Our findings suggest that in soft-sediment habitats deep-burrowing bivalves may be more resistant to otter predation than shallower burrowers.

Organochlorines and Mercury in Eggs of Coastal Terns and Herons in California, USA

-In San Francisco Bay, California, USA, concentrations of DDE and mercury in eggs differed among Caspian Tern, Forster's Tern, Black-crowned Night-Heron, and Snowy Egret in 1982. Geometric mean DDE concentrations were higher (P <0.05) in Caspian Tern eggs (6.93 ppm, wet weight) than in eggs of other species (1.92 2.84 ppm). Mean mercury concentrations were significantly greater in Caspian Tern (1.25 ppm) and Forster's Tern (0.90 ppm) eggs than in night-herons (0.41 ppm), but night-heron eggs contained higher concentrations of mercury than did the eggs of Snowy Egrets (0.21 ppm). There were no significant differences among species for mean concentrations of trans-nonachlor or PCBs; other organochlorines occurred in fewer than half of the samples, so means were not compared. Caspian Tern eggs from San Francisco Bay had higher PCB concentrations (4.85 ppm) than did eggs of this species from San Diego Bay, California (1.70 ppm) or Elkhorn Slough, California (1.83 ppm), but we detected no significant differences in mean concentrations of other organochlorines. DDE concentrations in 5 of 47 (10.6%) nightheron eggs from San Francisco Bay exceeded 8 ppm, a level associated with impaired reproduction in this species. DDE concentrations were negatively correlated with eggshell thickness in night-herons and egrets.