Sanctuary Integrated Monitoring Network
Monitoring Project

Understanding the effects of tidal wetland management on estuarine invertebrate assemblages in Elkhorn Slough

Principal Investigator(s)

  • Stacy Kim
    Moss Landing Marine Laboratories, California State University


  • SIMoN
Start Date: August 27, 2008

One of the most sensitive, best understood, and relevant indicators of estuarine ecosystem condition is the benthic infaunal assemblage. In a related project, data were recently collected by the expert Benthic Lab Research Team at Moss Landing Marine Laboratories from sites along the main Elkhorn Slough channel as part of a project funded by SIMoN (see other project). Such data are key to examining community-level changes since the last major sampling effort in the 1970s. Over the past 30 years Elkhorn Slough has experienced increased tidal flow, which has significantly altered the bottom of the main channel through the process of tidal erosion.

Most previous studies on benthic infauna have focused on the main channel of Elkhorn Slough. Very few data are available to compare benthic invertebrates at sites under different tidal management beyond the main channel. Indeed, there have been virtually no assessments of the benthic infauna in the wetlands managed by NOAA's Elkhorn Slough National Estuarine Research Reserve (e.g., South Marsh, North Marsh, Whistlestop Lagoon, Pick N Pull Marsh) or the Elkhorn Slough Foundation (e.g., Azevedo Ponds, Porter Marsh, Moro Cojo). Benthic infauna represent the characteristic estuarine assemblage that supports more familiar charismatic creatures such as migratory shorebirds, leopard sharks and sea otters. It is major reason why Elkhorn Slough serves as a major stop for migrating birds along the western US.

This project represents the first systematic and repeated sampling of benthic habitats beyond the main channel of Elkhorn Slough. These benthic invertebrate data will inform resource management decisions by ESNERR and ESF, and contribute to the Tidal Wetland Plan, which will govern future management of water movement within Elkhorn Slough.

Summary to Date

Researchers from Moss Landing Marine Laboratories sampled 19 stations in wetlands adjacent to the main channel of Elkhorn Slough during fieldwork in August 2007 and April 2008. One hundred eighty three infaunal cores and 113 sediment grain size cores were collected during the two sampling efforts. Samples are currently being processed (156 infauna and 36 grain size) and the rest have been archived for possible future work.

Monitoring Trends

  • Many of the field sites (see figure 1) harbored blooms of Ulva-like green macroalga during both sampling efforts. This macroalga often appeared to be layered, with the healthiest individuals floating on the water surface and those less healthy underneath them or on the bottom. The underlying layers appeared to be dying or dead, creating an anoxic layer of organic material covering and contributing to the sediment.
  • Lack of freshwater input and little tidal flushing resulted in many systems having salinities of 50 ppt or higher (sea water is 34 ppt) during the August 2007 sampling, which in conjunction with anoxic conditions appeared to limit diversity and abundance of visible macrofauna. In April 2008, after the rainy season, our impression of organisms on the sieve screens was very like the earlier sampling effort, with few individuals and few species represented in the infaunal community.
  • Common organisms included waterboatmen (Trichocorixa reticulata), the leptostracan Nebalia gerkenae, and Monocorophium spp. amphipods, as well as capitellid and spionid polychaetes. Ruppia maritima, a brackish water seagrass, was sparsely but widely distributed as well. Flowering and fruiting Ruppia maritima was collected in August 2007 and seedlings were found in multiple locations in April 2008.

Study Parameters

  • Distribution
  • Habitat association
  • Diversity
  • Habitat
  • Abundance
  • Disturbance
  • Substrate characterization

Study Methods

Infaunal sampling via 10 cm diameter x 15 cm deep cores; samples sieved over 0.5 mm screen, sorted, identified to species, and counted. Substrate characterization via grain size analysis of sediment cores. Habitat characterization by qualitative data collection and photography.

Figures and Images

Figure 1. Map of Elkhorn Slough tidal wetland sampling stations. Stations were sampled in August 2007 and April 2008.

Figure 2. John Oliver, Kamille Hammerstrom and Gage Dayton (single kayak) conduct a qualitative survey of the Moro Cojo benthic community on August 21, 2007.

Figure 3. John Oliver and Peter Slattery discuss the identity of a crustacean from an infaunal sample in East Bennett Slough on August 17, 2007.


  • Oliver et al. (2009)
    Benthic invertebrate communities in the peripheral wetlands of Elkhorn Slough, ranging from very restricted to well flushed by tides
    3.9 MB PDF