Phytoplankton, Biodiversity, and Invasive Species in Elkhorn Slough

Elkhorn Slough is well known for its rich flora and fauna, especially as represented by larger organisms that form conspicuous populations along the slough wetlands. However, the planktonic biota of Elkhorn Slough have not been so well characterized. As part of the Monterey Bay National Marine Sanctuary’s SIMoN (Sanctuary Integrated Monitoring Network) project, scientists at Moss Landing Marine Laboratories (MLML) have begun a quantitative assessment of plankton community structure and function within Elkhorn Slough. One of SIMoN’s primary missions is to document past and present sanctuary ecosystem characteristics for the purpose of monitoring environmental change. We report here our first observations addressing the phytoplankton of the slough.

Ten sampling stations were established along the main channel of the slough from the mouth near the Moss Landing Harbor (Station 1) to the upper section past Kirby Park (Station 10). More than fifty sampling trips have now been completed during the first year (2002/2003) of our study, using small boat operations available at MLML. Using chemical (chromatographic) separation techniques, we analyzed water samples for phytoplankton pigments (chlorophylls and carotenoids), some of which by their presence indicate the occurrence of specific algal taxa. The taxon-specific pigments, discussed here, provide an indication of the algal diversity of the slough’s phytoplankton community.

The information acquired so far has proven quite interesting. Without exception, all results have shown that the Elkhorn Slough phytoplankton community is divided into two assemblages: lower slough and upper slough (Figures 2a and b).

The lower slough, near Monterey Bay, is dominated by a rich diversity of coastal phytoplankton including dinoflagellates, cyanophytes, green algae, and most importantly, diatoms. The community effectively reflects the rich composition of phytoplankton found in the Monterey Bay source waters. The diatoms are easily tracked by their dominant carotenoid, fucoxanthin (the same carotenoid found in brown seaweeds). The upper end of the slough, inland of Parson’s Slough, is characterized by high concentrations of alloxanthin, a carotenoid found only in the small algal group, Cryptophyta (less than twenty genera of cryptophytes are known). Cryptophytes are often present in Monterey Bay, however their con- centrations are usually relatively low. Cryptophytes at the upper end of Elkhorn Slough, near Kirby Park, can make up more than 75 percent of all the phytoplankton biomass. Microscopic analysis shows that the planktonic flora of upper Elkhorn Slough is relatively species poor, dominated by only one or two species of cryptophyte.

To our knowledge, the division of Elkhorn Slough phytoplankton into two communities has not been previously known. However, all new observations suggest that the division is surprisingly persistent. For instance, the interface between the upper and lower algal communities can be seen to ebb and flood with the tide, without loss of resolution due to horizontal mixing. The community division has persisted through all four seasons sampled so far, including the high runoff period in winter. We are not sure how long the conspicuous phytoplankton community division has existed in Elkhorn Slough, but we are attempting to reconstruct that history through examination of the sediments.

The observation that Elkhorn Slough phytoplankton are grossly divided into a richly diverse bay-ward community and a relatively species-poor upper slough community poses interesting questions regarding the recent observations of introduced invasive species in the slough. Researchers at the Elkhorn Slough Foundation have listed more than fifty species of introduced marine invertebrates in the slough; interestingly, the most severe invasive conditions are noted in the upper slough. Most of these invasive invertebrates spend part of their early life within the planktonic habitat, and many feed on planktonic organisms after they settle as adults. One wonders whether the growing biomass of introduced benthic filter feeders in the upper slough could selectively remove the larger algal cells (e.g., coastal diatoms, dinoflagellates) originating from Monterey Bay, leaving small (<10 µ) cryptophytes to proliferate.

We have begun analyzing core samples from Elkhorn Slough sediments where the algal cells (and their carotenoids) settle and leave taxonomic markers of phytoplankton activity through time. Surface sediments are clearly tagged with the same distributional pattern of fucoxanthin and alloxanthin found in the water column. However, alloxanthin in the upper slough can be detected at least thirty centimeters into the sediment core, suggesting that the separation of phytoplankton communities may have existed even before the recent observations of invasive species, made only within the last fifteen years.

Numerous theoretical and experimental observations have suggested that species-poor systems are inherently unstable and possibly more prone to species invasions. Could the structure of phytoplankton community diversity through Elkhorn Slough affect the likelihood of new species invasions for organisms that exploit the planktonic habitat through various stages of their lives? Con-tinued work will focus on understanding the processes (biological, chemical, and physical) that result in the maintenance of distinct Elkhorn Slough phytoplankton distributions. Possibly a relation-ship between the observed invertebrate species invasions and phytoplankton biodiversity will evolve.

Nick Welschmeyer, Lawrence Younan, Andrew Thurber,
and Gala Wagner
Moss Landing Marine Laboratories