|
With time, we will determine whether differences in response between zooplankton and fish to restoration represent simply a "trophic delay" such that lower trophic level organisms respond more quickly to changed conditions than higher level ones. While waterfowl responded almost immediately to restoration of the Project Area, arriving in high numbers only a month after the levees were breached, shorebird numbers the first winter were actually quite low, although they started to climb somewhat during the second fall/winter season and third fall (See Bird Web Page article). Avian biologists directly attributed low numbers of shorebirds immediately after restoration to a low invertebrate prey base (J. Kelly, Audubon Canyon Ranch, pers. comm., J. Evens, ARA, pers. comm.). In addition, the type of shorebirds that showed up during the early years were species that forage on the surface (e,g., Black-bellied plovers), shallow probers (e.g., Least sandpipers), and those that forage in the shallow water column (e.g., Greater yellowlegs; J. Evens, ARA, pers. comm.). Benthic invertebrate data suggests that numbers of benthic invertebrates within the restored wetlands are increasing, and this is perhaps borne out by the fact that, in fall 2011, Marbled godwits arrived for the first time in moderate numbers at the wetlands, signaling the presence of deeper substrate probers (J. Evens, ARA, pers. comm.). Interestingly, results from fish and invertebrate monitoring at the restored Giacomini Wetlands are almost diametrically opposite of those from other studies of tidal wetland restoration projects. In those projects, fish numbers and species composition typically changed dramatically in the first couple of years after restoration, often quickly resembling the structure observed in reference marshes (Williams and Zedler 1999, Konisky et al. 2006, Warren at al. 2002, Desmond et al. 2002, Havens et al. 2002, Raposa and Roman 2003). Rather than being driven by restoration or native marsh status or even age of restoration, several projects found that differences between fish sampling sites more strongly depended on differences in physical structure of the marsh channels (e.g., width, depth, slope of bank, marsh elevation), as well as hydrologic and environmental conditions such as hyroperiod, water temperature, dissolved oxygen (D.O.), salinity, discharge, etc. (Minello and Webb 1997, Williams and Zedler 1999, Desmond et al. 2002, Havens et al. 2002, Raposa and Roman 2003). Where marsh status (restored vs. native) and age of restored marsh really seemed to play a substantial role was in benthic and epibenthic invertebrate communities. Many studies done to date have found substantial differences in at least some invertebrate parameters between restored and reference marshes, even after 10 to 20 years post-restoration (Minello and Zimmerman 1992, Minello and Webb 1997, Talley and Levin 1999; Ferguson and Rakocinski 2008). Almost all sites sampled showed differences in either densities or community structure between constructed and natural marshes within the first few years after construction (Cammen 1976; Minello and Zimmerman 1992; Scatolini and Zedler 1996; Levin et al. 1996), although there were a few exceptions. Sieperda Marsh in the Scheldt Estuary in the Netherlands developed macrofauna typical of reference estuarine mudflats within five (5) years (Eertman et al. 2002). In some ways, however, results from studies evaluating evolution of benthic invertebrate communities are somewhat biased by the method used to construct or restore these marshes. Almost every one of the studies referenced above that compared benthic communities between constructed and natural marshes used sites that were built from dredge spoil material, which was one of the earliest tidal marsh restoration techniques. Needless to say, dependent on the source of dredge spoil material, the likelihood that marsh substrate would be similar between constructed and natural systems--even if they are directly adjacent--is extremely low, and so, therefore, should be the expectation that these systems would develop similar benthic invertebrate communities. Sand is frequently a readily available source of dredge spoil material, as it is dredged from shipping lanes and harbors and was certainly used in many of the early restoration or construction projects (e.g., Sweetwater Marsh in San Diego Bay). Sand is also likely to support an entirely different benthic invertebrate community than estuarine muds, which is the substrate of many natural marshes. Interestingly, some of the sites where benthic invertebrate communities evolved more quickly were ones where dikes were breached (Sieperda Tidal Marsh; Eertman et al. 2002) or uplands were excavated (Sarah's Creek; Havens et al. 2002), although some culvert replacement where substrate wasn't necessarily disturbed still had invertebrate communities that had not converged with reference marsh ones (Warren at al. 2002). The speed with which benthic invertebrate communities in the Giacomini Wetlands have shifted with restoration may result largely from the fact that this was a levee breach project where the substrate has largely remained intact, with a few exceptions. It would be interesting to determine if the trajectory for areas where excavation was performed to create creeks and marshplains within the Project Area differs from those areas where no excavation was performed. In addition, as noted with benthic invertebrate results, some of the change observed may not be related to restoration at all. Variation in annual precipitation could have inadvertently affected results, with the first two years after restoration being drier in terms of lower rainfall than many of the years prior to the levees being breached. Heavy rainfall conditions can greatly impact benthic invertebrate communities, in particular (Nordby and Zedler 1991), so increases in benthic invertebrate densities in both the Project Area and Reference after restoration may be due to more homogenous salinity conditions during the relatively lower-rainfall years. The effect of drier conditions on zooplankton densities was harder to interpret, due to the substantial spatial and temporal variation in densities between sampling sites in all the Study Areas. Yet another interesting implication of the results was the large number of non-native species present in both the Project Area and Reference Areas. While relative abundance of non-native fish species declined in the Project Area with restoration, a considerable number of the benthic invertebrate, as well as zooplankton, species are non-native, and some of those that are native are actually earlier introduced species that are now considered naturalized. These non-native species introduce a wild-card factor into the evolution of the newly restored Giacomini Wetlands. Yes, the restored wetlands are becoming more similar to the reference marshes, but that may not mean much if the food chain in Tomales Bay is disrupted by competition from--and even displacement by--non-native invertebrates and fish. As has been seen in San Francisco Bay, these non-native species can play havoc with the entire food web of an estuary and could ultimately drive certain listed or commercially important species towards extinction. Untangling the Food Web: Changes in Prey Base Following Restoration |
Last updated: September 18, 2022