– – – – – – – –
BY KERSTIN WASSON

November 2003: Intertidal mudflats are rich habitats, hosting a wealth of invertebrates that in turn support migratory shorebirds, foraging fish including sharks and rays, and marine mammals such as harbor seals and sea otters. Estuaries are few and far between in California, and many estuaries that originally had extensive mudflats have been degraded by urbanization. Elkhorn Slough boasts some of the richest and most extensive mudflat communities remaining in the state.

In 1946 the Army Corps of Engineers cut a new channel to the mouth of Elkhorn Slough (click here to see a larger view), which increased its exposure to a form of erosion called "tidal scour."

Are the mudflat communities of Elkhorn Slough still healthy and diverse? Recently, the 2003 Elkhorn Slough Conservation Research Award was presented to Katherine Fenn in honor of her work addressing this very question. She documented dramatic changes in natural communities occurring since the 1970s, involving changes in species composition and abundance, and suggests they may be the result of erosion of mudflat habitats caused by creation of an artificial harbor mouth. However, she found no decrease in native species richness, and no increase in non-native species abundance in these mudflat communities – reassuring news for an estuary subject to pollution and invasions by exotic species.

In the 1920s, George MacGinitie, carried out seminal descriptive research on Elkhorn Slough invertebrates. But his surveys were not quantitative, and unfortunately there is no real baseline of what communities were like in his day, other than which species he happened to collect and describe. We will never know how these communities were altered in subsequent decades by factors such as the 1946 opening of an artificial wide mouth to the estuary to accommodate the newly created Moss Landing Harbor, increased agricultural pollution in the watershed, and construction of the nearby Moss Landing power plant, because there are simply no “before” data for a “before/after” comparison.

The telltale "volcano" burrow outlet of a fat inkeeper worm, one of the larger invertebrates of the intertidal mudflats.

However, in the 1970s, researchers at Moss Landing Marine Laboratories (MLML) carried out the first quantitative assessments of mudflat communities, providing a baseline for future studies. Ms. Fenn repeated and expanded the sampling regime developed by these researchers, with mentoring from the original MLML team. She collected coffee-can cores of mud along transects at four stations in the main Slough channel, then sieved, sorted, identified and counted all the invertebrates she found. Fenn, who now works as steward for the Natural Reserves at the Santa Cruz campus of the University of California, did this work as a part of her Master’s thesis at Miami University in Ohio.

Her results revealed a statistically significant shift in taxonomic composition and abundance patterns of invertebrate communities between the 1970s and the present. To what can these changes be attributed? Fenn reviewed three factors that could be driving changes: exotic species, water quality, and tidal erosion. She concluded that evidence for the latter was most compelling. In these decades, habitat change due to tidal erosion appears to have altered conditions for invertebrates much more dramatically than the other two factors.

Researchers conduct a "transect" of the mudflats, counting invertebrates along a known line.

The mudflat changes that occurred between the 1970s and the present are of concern, because they suggest that human development has altered invertebrate communities and therefore food webs. However, other results were more reassuring. There were few significant differences in invertebrate communities between surveys in the 1990s and 2001, possibly indicating that rates of change may have slowed in the last decade. Also, the total number of species collected in 2001 was similar to data from the 1970s, so by this index, biodiversity has not decreased. Indeed, Fenn’s results reveal that invertebrate communities are still rich in species, despite the threats from pollution, power plant intake, tidal erosion, harvesting, and other human activities. In addition, Fenn found no significant pattern of increase in the variety or abundance of exotic species over time.

Fenn’s work will help to improve the quality and focus of future monitoring studies, including studies currently being done by MLML, funded by the Monterey Bay National Marine Sanctuary. Fenn’s results revealed very patchy distributions of invertebrate species. She sampled at three different tidal heights (0.0, -0.2, and –0.4 m below mean low water) and found that at a given site, a single animal group (e.g., worm, bivalve, crustacean) dominated a given tidal height. However, the particular group varied between sites. Therefore, a comprehensive taxonomic survey would only be obtained by sampling at all three tidal heights at all sites. Past monitoring studies were only done at one tidal height. She also found startling alongshore variation in community composition: cores along the same transect only one meter apart were often radically different. This suggests that to adequately characterize a large site, more samples must be taken along the shoreline than had been done previously.

The Elkhorn Slough Conservation Research Award is given to a junior researcher whose investigations inform estuarine conservation. The award is sponsored by the Elkhorn Slough Foundation (ESF) and the Elkhorn Slough National Estuarine Research Reserve (ESNERR), which is owned and managed by the California Department of Fish and Game, in partnership with NOAA and ESF. ESF and ESNERR practice science-based management of Elkhorn Slough and its watershed, and support applied conservation research. Each year, dozens of local students, faculty, and other researchers complete short-term scientific investigations at Elkhorn Slough, which complement the long-term monitoring programs coordinated by ESF and ESNERR staff.