The delta restoration project introduced tides to the Nisqually estuary for the first time in over a century, changing vegetation communities, channel morphology and water quality. The physical changes presented new opportunities for salmon to use restored habitats as they migrated from the freshwater to saltwater. Yet the success of this restoration cannot be determined based on the habitat opportunities alone–scientists must also examine the capacity of the new habitats to provide food for juvenile salmon. By sampling the invertebrate community structure, biodiversity, key species densities and overall biomass, scientists are able to get a broader view of the impacts of the Nisqually Delta Restoration on Chinook salmon and other fish.
In general, different salmon species prey on varying invertebrates: Chinook salmon typically eat arthropods–like flies and ants–that fall into the water from surrounding vegetation. On the other hand, chum salmon prefer planktonic invertebrates and small crustaceans found in the water. To complicate matters, salmon species migrate at different times of the year. Therefore, salmon foraging capacity is closely linked to timing of available prey, not just the variety.
Scientists from the Nisqually Indian Tribe and USGS collected three types of invertebrate samples: benthic, terrestrial, and planktonic. Benthic invertebrates are small creatures that live in the rocks and sand of waterbodies–examples include stoneflies, mayflies and dragonflies. These were monitored in August or September 2009-2012. Terrestrial invertebrates, including flies and beetles, were monitored in April-July of 2009-2012. Finally, planktonic invertebrates were monitored April-July of 2009-2012.
Additional information collected as a part of this monitoring effort included soil and water quality information and fish gut contents. The soil and water quality data was collected in conjunction with the benthic invertebrate sampling. Fish gut contents verified Chinook diets. In all, 118 fish gut samples were collected in April-July of 2012.
Because certain invertebrates are sensitive to particular physical parameters, measuring soil and water quality changes helped scientists better understand and predict the driving forces behind invertebrate communities. For example, some invertebrates live in freshwater habitats; when tides returned to the restored area, the available freshwater habitat disappeared. To understand the physical variables that influence invertebrates, scientists analyzed organic matter, soil composition (i.e., % sand, silt or clay), water temperature, dissolved oxygen (DO), salinity and percent vegetation cover.
The data collected suggests that changes to physical parameters could sometimes be linked to the distance from open water and the sampling site. Vegetation cover decreased rapidly across all Refuge sites in response to the influence of saltwater. In fact, vegetation cover dropped from 91.5% pre-restoration to 1% post-restoration. This led to a decrease in organic matter on the restoration sites. The soil and water quality, as well as vegetation cover, was a key driver behind the invertebrate communities post-restoration. The elevation of the restoration area directly influenced vegetation communities. Water quality parameters, such as salinity, also influenced vegetation and invertebrate communities.
After collecting all samples, scientists calculated community structure, biodiversity, density and available biomass of benthic invertebrates at different locations throughout the Nisqually delta. The data showed that significant invertebrate community changes occurred only on Refuge–not reference–sites, and were primarily influenced by salinity changes.
After tides returned to the area, salinity changes influenced each site differently. Unit 1, for instance, had a 150% increase in salinity while Unit 3 had an 80% increase. The increase in salt had a direct impact on benthic invertebrates, with the greatest community changes occurring at western sites, and those most prone to tidal inundations.
The benthic invertebrate communities showed a notable increase in polychaete, arthropod and nematode densities through time. For instance, Polychaete densities increased from 0 to over 12,000 individuals per meter squared.
To monitor changes in terrestrial invertebrate communities, scientists installed fallout traps to collect insects that fell from vegetation into the water. The traps were installed at 4 sites: Madrone, Animal, Phase II and Control Sloughs. The traps were in place April through July of 2009-2012, and collected 96 samples per year.
Terrestrial invertebrate communities changed after the restoration, and were closely linked to the elevation and vegetation cover of each monitoring unit. In fact, species richness correlated positively with vegetative cover, suggesting that as vegetation cover continues to increase at restored sites, so will the numbers of terrestrial invertebrates. Furthermore, invertebrates that can move quickly, like beetles, flies or true bugs, are steadily increasing in abundance. This is most likely because of their ability to fly in and out of suitable habitat. Less mobile species, like spiders, are expected to increase in abundance as time progresses and as vegetation regrows.
The monitoring results showed large differences between terrestrial communities at Refuge sites (restored in 2009) and those on Tribal lands (restored in 2006). In particular, sites on Refuge lands are recovering more slowly than those on Tribal lands. Lands restored in 2006 are at a higher elevation than those restored in 2009, and did not experience the same loss of vegetation. In contrast, the sites restored in 2009 had a large transition from grassy fields to mudflat habitat. This resulted in the disappearance of many invertebrates, followed by a slow recovery linked to vegetation recovery.
The delta restoration has the potential to enhance salmon habitat at the Nisqually River Delta. However, the available habitat is only a small part of the components necessary for salmon recovery. Food sources are vital too. The Nisqually River supports several different species of salmon, all of which migrate at different times of the year, and prefer different invertebrate species. This study has helped scientists understand what invertebrates are present during different times of the year. The information will help scientists understand more fully the capacity of the Nisqually River Delta in supporting various salmon species.
In an even greater context, the invertebrate communities across the delta form the basis of the food chain for many migratory bird species. Understanding the types of insects and invertebrates present during key foraging seasons helps land managers better understand the ability of the delta to provide food for bird species.