Changes in Elkhorn Slough mudflat communities over time

Are the mudflat communities of Elkhorn Slough healthy and diverse? Have they changed over time? Local UCSC Researcher Katherine (“Tabby”) Fenn recently shed light on these questions, and for her contributions she will be honored with the 2003 Elkhorn Slough Conservation Research Award.

Fenn sampled mudflat invertebrates – indicators of estuarine ecosystem health, and food for shorebirds, sea otters, and leopard sharks. She documented dramatic community changes since studies by researchers at Moss Landing Marine Laboratories in the 1970s, and suggests they may be the result of erosion of mudflat habitats caused by the artificial harbor mouth. However, she found no decrease in native species richness, and no increase in non-native species in these mudflat communities– reassuring news for an estuary subject to pollution and invasions by exotic species.

Intertidal mudflats are rare in California, and Elkhorn Slough boasts some of the most diverse and extensive mudflat communities in the state. Mudflat invertebrates also form an integral part of the food web, supporting migratory shorebirds and foraging sea otters in the Slough.

In the 1920s, George MacGinitie, carried out seminal descriptive research on the spineless wonders of the Slough. 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 opening of an artificial wide mouth to the estuary to accommodate the new Moss Landing harbor, increased agricultural pollution, and construction of the Moss Landing power plant, because there are simply no “before” data for a “before/after” comparison.

However in the 1970s, researchers at Moss Landing Marine Laboratories (MLML) , carried out the first quantitative assessments of mudflat communities, setting a baseline for future studies. The researchers involved with this were members of the Invertebrate Zoology and and Benthic Ecology Groups: James Nybakken, Chris Jong, John Oliver, Peter Slattery. Another MLML team lead by Rikk Kvitek (now at California State University Monterey Bay) repeated these surveys in the early 1990s.

Tabby Fenn recently decided to repeat and expand the sampling regime developed by these researchers, with mentoring from the MLML team. Like them, 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 tiny invertebrates she found. J. Oliver and P. Slattery at MLML generously provided laboratory space, supplies, and taxonomic assistance for her study. C. Jong, now at UCSC, provided the raw data from the 1970s surveys – which others assumed had been lost, yet which was critical for the analyses.

Fenn, who is now the Reserve Steward for the UCSC Natural Reserves, carried out this work as a part of her Master’s thesis at Miami University in Ohio. “I didn’t know much about marine biology when I initiated my research, and I knew even less about benthic ecology,” said Fenn, whose past research had included rare birds in the Florida everglades. “However the project was appealing to me because of the availability of long-term data. Many of the organizations I have worked for have a wealth of archived data, but staff biologists have so many current responsibilities that revisiting old datasets is almost impossible. I wanted to tackle that job for a conservation organization and thereby help inform decisions regarding habitat conservation and wise stewardship of natural areas.”

A statistical analysis of her results revealed a significant shift in invertebrate communities between the 1970s and the present. To what can these changes be attributed? According to Fenn, “I reviewed evidence concerning three factors that could be drive such a change: exotic species, water quality, and tidal erosion. I found little to support the first two factors. Evidence for the third factor was most convincing”. Fenn suspects that tidal erosion (resulting from harbor development) is a key driving force, because it is the one that has changed physical conditions the most in these decades.

“The changes between the 1970s and the present are of concern, because they may indicate that human influences are altering invertebrate communities, and thus Slough food webs,” notes Dr. Kerstin Wasson, research coordinator at the Elkhorn Slough National Estuarine Research Reserve. However, she points out that Fenn’s results were also reassuring. There were few significant differences between data in the 1990s and 2001, so rates of change may have slowed down. Also, there was no difference in total number of species collected in the 1970s vs. 2001, so by this index, biodiversity has not decreased. Indeed, Wasson says, “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 the area.” In addition, Fenn found no significant pattern of increase of exotic species numbers or abundance over time – a pleasant surprise in a very heavily invaded estuary.

The sampling carried out by Fenn will also inform future estuarine monitoring efforts. Fenn’s results revealed very patchy distributions of the invertebrate species. She sampled at three tidal heights, and found that within a site, one animal group dominated a given tidal height. However, the identity of the group (worm, bivalve, crustacean) varied across sites. Therefore a comprehensive view of all groups can only be obtained by sampling at all three tidal heights at all sites – while the past monitoring was only carried out at one height. She also found startling alongshore variation – cores along the same transect only 1m apart were often radically different. This suggests that to adequately characterize a large site, more samples must be taken along the shoreline than has been done in the past. Her work will improve the caliber of future monitoring, including current efforts being developed and implemented at MLML with funding from the Monterey Bay National Marine Sanctuary.

The Elkhorn Slough Conservation Research Award is given to a junior researcher whose investigations of this estuary provide information critical for wise conservation of Slough ecosystems. The award is jointly 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. Both ESF and ESNERR practice science-based management of Elkhorn Slough and its watershed, and strongly 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.

contact information:

Katherine (Tabby) Fenn
Natural Reserve Steward
UCSC Natural Reserve Office
phone: 831 459-4971

Dr. Kerstin Wasson
phone: 831 728 5939 (W)

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