Wetland Availability and Salinity Concentrations for Breeding Waterfowl in Suisun Marsh, California

Abstract

Availability of wetlands with low salinities during the breeding season can influence waterfowl reproductive success and population recruitment. Salinities as low as 2 ppt (3.6 mScm–1) can impair duckling growth and influence behavior, with mortality occurring above 9 ppt (14.8 mScm–1). We used satellite imagery to quantify the amount of available water, and sampled surface water salinity at Grizzly Island, in the brackish Suisun Marsh, at three time-periods during waterfowl breeding (April, May, July) over 4 years (2016–2019). More water was available and salinity was lower during wetter years (2017, 2019) than during drier years (2016, 2018), and the amount of water in wetlands decreased 73%–86% from April to July. Across all time-periods and years, the majority (64%–100%) of wetland habitat area had salinities above what has been shown to negatively affect ducklings (> 2 ppt), and up to 42% of wetland area had salinities associated with duckling mortality (> 9 ppt). During peak duckling production in May, 81%–95% of available water had salinity above 2 ppt, and 5%–21% was above 9 ppt. In May of the driest year (2016), only 0.5 km2 of low-salinity water (< 2 ppt) was available to ducklings in the study area, compared to 2.6 km2 in May of the wettest year (2017). Private duck clubs own the majority of wetland habitat at Grizzly Island and consistently had a greater percentage of land flooded during summer than did publicly owned wetlands, but private wetlands generally had higher salinities than public wetlands, likely because they draw from higher-salinity water sources. By July, few wetlands remained flooded, and most had salinities high enough to impair duckling growth and survival. Local waterfowl populations would benefit from management practices that provide fresher water during peak duckling production in May and retain more water through July.

Predator movements in relation to habitat features reveal vulnerability of duck nests to predation

“…Phragmites and shrub patches were typically large enough to be visible in satellite imagery (>5 m in diameter), but we also used real‐time kinematic GPS (Leica Smart Rover GPS1200, Leica Geosystems) to ground truth the edge of these smaller habitat features that may have less visible using satellite imagery. The digitizing of water features is described in detail within Schacter et al (2021). Briefly, for seasonal wetlands we used satellite‐derived imagery to digitize the extent of water at three times during the waterfowl nesting period for each year: April (early nesting), May (mid nesting), and July (late nesting).…”

The Anatomy of a Drought in the Upper San Francisco Estuary: Water Quality and Lower-Trophic Responses to Multi-Year Droughts Over a Long-Term Record (1975-2021)

“…The effect of drought periods was strongest in the westernmost regions (as much as 6 PSU in Suisun Bay and Suisun Marsh), which was also expected because these are the most seaward regions and are more subject to salinity increases. Salinity increases of 6 PSU could completely shift phytoplankton and zooplankton communities in Suisun Bay (Cloern and Jassby 2012), limit Delta Smelt habitat (Bever et al 2016), reduce nesting habitat for birds in Suisun Marsh (Schacter et al 2021), and could increase the upstream distribution of invasive clams and jellyfish (Hartman et al, this issue).…”

Data to Decisions: How to Make Science More Relevant for Management of the San Francisco Estuary

“…Management-relevant science that is foundational research enhances the knowledge base on a topic or aspect of the estuary that may influence decisionmaking but is not necessarily linked to a specific regulation or management action. An example of this foundational research may be a description of salinity patterns in wetlands and the associated species composition of fish or invertebrates across salinity regimes (e.g., Schacter et al 2021;Young et al 2018), which may then indirectly inform potential outcomes of specific management actions such as restoration. This type of management-relevant science may be broad, ranging from a characterization of contaminant concentrations across the estuary, to development of genetic tools to identify species presence more promptly or accurately, to synthesis work to examine trends in water temperature.…”