Liberty Island Provides Insights Into Delta Ecosystem Restoration

Liberty Island in the Cache Slough complex of the north Delta appears to provide key habitat for delta smelt and other native fish species. Today, research is being done to examine and learn from the success of the island’s accidental launch into ecosystem restoration-after a levee breach more than a decade ago.

Recent studies have revealed several important observations in the Cache Slough complex:

  • It appears that delta smelt are utilizing the near-shore habitats of Liberty Island not only during the spawning season, but on a year-round basis;

  • Turbidity is an especially important component of habitat for Delta pelagic fishes;

  • Much of the sediment and organic carbon flux in the estuary passes through the Cache Slough complex; and

  • The Cache Slough complex may be an important “food bank” for the Delta.

Therefore, understanding the patterns of hydrodynamics, turbidity, phytoplankton, zooplankton, marsh vegetation, and food web interactions is important to describe the habitat for pelagic fishes including delta smelt, and could help to guide tidal marsh restoration efforts.

BREACH III Study

One interdisciplinary study, BREACH III: Evaluating and Predicting ‘Restoration Thresholds’ in Evolving Freshwater-Tidal Marshes is led by BREACH studies team leader Charles (Si) Simenstad of the University of Washington. An important objective of the BREACH III study-the third on the effects of accidental levee breaches-is to understand hydrologic and geomorphic changes and the ecological response in restoring wetlands at the landscape scale. Another goal is to develop a measurement and predictive tool to guide future restoration to successful outcomes.

“We are hoping to build a predictive level of understanding about (1) how abiotic and biotic factors in a restoring tidal marsh (Liberty Island) control vegetation colonization and expansion and subsequent responses by native fish and wildlife, and (2) how restoration processes influence local flooding and levee erosion over the course of the restoration,” said Pete Hrodey, a fish biologist at U.S. Fish and Wildlife Service and the lead agency coordinator on BREACH.

“The researchers and scientists working in and around Liberty Island as part of the CALFED Ecosystem Restoration Program (ERP) that funded BREACH III hope to inform the decision-making process of resource managers and policy makers in charge of proposed restoration efforts in the area,” Hrodey said. “The Cache Slough complex, and Liberty Island in particular, has been recognized by the Interagency Ecological Program (IEP), CALFED, Bay Delta Conservation Plan and others as potential refugia for delta smelt, splittail, salmon, and other native fish species and for its hydrodynamic influence within the north delta region.” During 2002-2005 the Fish and Wildlife Service was able to collect various life stages of delta smelt within Liberty Island on regular time intervals.

“Based on those data and our current monitoring efforts at Liberty Island,” Hrodey said, “it appears that delta smelt are utilizing the near-shore habitats of Liberty Island not only during the spawning season, but on a year-round basis. Because of these findings, resource managers and regional planners have touted Liberty Island as a model for tidal marsh restoration, yet we still don’t know what it is about Liberty Island that makes it so attractive to delta smelt. Our work with Breach III will try to address some of the uncertainties surrounding habitat use by various life stages of delta smelt and other species of concern within Liberty Island. The results of this work will hopefully help to guide tidal marsh restoration efforts and the design of specific projects hoping to mimic those habitat conditions in Liberty Island that favor delta smelt and other native species.”

“The research is specifically designed to examine, and hopefully come up with a predictive capacity to prescribe the scientific and technical processes that are required for better management decisions,” Simenstad said.

CALFED ERP funded two earlier BREACH studies-one comparing breached levee sites in the Delta, and one examining sites further downstream in Suisun and San Pablo bays. The goal of that research was to analyze historically-breached levee wetlands, in comparison to intact “reference” sites, as a means to predict the feasibility, patterns, and rates of restoration to natural ecological function. To learn about those studies and some of the findings, click here.

IEP Cache Slough Studies

The IEP is currently examining the hydrodynamic “footprint” of Liberty Island. Below are the preliminary (still unpublished) results of the ongoing IEP studies, provided by Ted Sommer of the Department of Water Resources (DWR):

Turbidity (Effort led by Dave Schoelhammer and Tara Morgan-King, U.S. Geological Survey)

  • Turbidity in this area seems to be controlled by two things: big inflow events and wind.

  • Seasonally, big inflow events such as the flooding of Yolo Bypass bring big sediment pulses into the region.

  • Overall, the region is remarkably turbid because of windy conditions that stir up lots of sediment.

  • Why This is Important:

    • Turbidity has been dropping a lot in the Delta due to reduced sediment inputs from upstream and the “filtering” effect of expanding beds of aquatic plants.

    • Pelagic native fish like delta smelt need turbid water.

    • A working hypothesis is that the resulting high turbidity in Liberty Island (and the broader region) is a key reason why the area has become a “hot spot” for delta smelt.

Food (Effort led by Gina Benigno and Ted Sommer, DWR)

  • Food availability (phytoplankton, zooplankton) varies a lot with tide in the Cache Slough region.

  • Zooplankton densities are relatively high in the Cache Slough region compared to many other parts of the Delta.

  • Phytoplankton levels aren’t remarkably high in Liberty Island itself, but are at impressive levels in the smaller sloughs that surround the island.

  • Why This is Important:

    • Phytoplankton and zooplankton have declined in the Delta, likely reducing food availability for imperiled species like delta smelt.

    • There is some evidence that some of the variability in delta smelt populations is due to zooplankton availability.

    • Higher zooplankton abundance in Liberty Island likely makes the region attractive to delta smelt.

    • A hypothesis is that phytoplankton transported from the smaller channels around Liberty Island act as a “food bank” for zooplankton in the region.

“The Cache Slough complex (including Liberty Island and the Deep Water Ship Channel) seems to provide decent habitat for delta smelt and other fishes because it is turbid and has more food,” Sommer said, adding that they would continue to flesh out this story as the data are analyzed.