Avian response to early tidal salt marsh restoration at former commercial salt evaporation ponds in San Francisco Bay, California, USA

Restoration of former commercial salt evaporation ponds in the San Francisco Bay estuary is intended to reverse a severe decline (\u3e79%) in tidal salt marshes. San Francisco Bay is a critical migratory stopover site and wintering area for shorebirds and waterfowl, and salt ponds are important high tide roosting and foraging areas. Conservation of past bird abundance is a stated goal of area restoration projects, and early adaptive management will be critical for achieving this objective. However, initial avian response at sites restored to tidal flow may not be indicative of long-term results. For example, winter shorebirds at a 529 ha pond breached in 2002 showed a marked increase in shorebird abundance following breaching. Shorebirds comprised 1% of area totals during 1999-2002 and increased to 46% during 2003-2008. These changes accompanied increased tidal range and sedimentation, but minimal vegetation establishment. Conversely, a fully vegetated, restored 216 ha pond in the same system consistently supported less than 2% of all waterbirds in the region. Early restoration may temporarily increase habitat, but managed ponds will be needed for long-term waterbird abundance within a restored pond-marsh system

Unintended consequences of management actions in salt pond restoration: cascading effects in trophic interactions.

Salt evaporation ponds have played an important role as habitat for migratory waterbirds across the world, however, efforts to restore and manage these habitats to maximize their conservation value has proven to be challenging. For example, salinity reduction has been a goal for restoring and managing former salt evaporation ponds to support waterbirds in the South Bay Salt Pond Restoration Project in San Francisco Bay, California, USA. Here, we describe a case study of unexpected consequences of a low-dissolved oxygen (DO) event on trophic interactions in a salt pond system following management actions to reduce salinity concentrations. We document the ramifications of an anoxic event in water quality including salinity, DO, and temperature, and in the response of the biota including prey fish biomass, numerical response by California Gulls (Larus californicus), and chick survival of Forster's Tern (Sterna forsteri). Management actions intended to protect receiving waters resulted in decreased DO concentrations that collapsed to zero for ≥ 4 consecutive days, resulting in an extensive fish kill. DO depletion likely resulted from an algal bloom that arose following transition of the pond system from high to low salinity as respiration and decomposition outpaced photosynthetic production. We measured a ≥ 6-fold increase in biomass of fish dropped on the levee by foraging avian predators compared with weeks prior to and following the low-DO event. California Gulls rapidly responded to the availability of aerobically-stressed and vulnerable fish and increased in abundance by two orders of magnitude. Mark-recapture analysis of 254 Forster's Tern chicks indicated that their survival declined substantially following the increase in gull abundance. Thus, management actions to reduce salinity concentrations resulted in cascading effects in trophic interactions that serves as a cautionary tale illustrating the importance of understanding the interaction of water quality and trophic structure when managing restoration of salt ponds

Study location in South San Francisco Bay, California, Alviso Pond A17 (north) and Pond A16 (south).

<p>Arrows indicate the dominant direction of flow from Coyote Creek in the north through the inflow gate to Pond A17, through the uncontrolled internal levee channel into Pond A16, and out the A16 discharge gate in the southeast corner into Alviso Slough. The circle indicates datasonde location; triangles indicate minisonde sampling locations. Forster’s Tern nesting colonies occurred on the four linear islands in the southern part of pond A16. Imagery provided by Landsat 8 and 2005 National Agricultural Imagery Program (NAIP).</p

Monthly mean salinity (PSU; ± SE; <i>n</i> = 4) changes over the course of restoration in Pond A16.

<p>We used four measurements from around the pond to calculate SE where August 2005 corresponds with a period of hypoxia.</p

Daily survival rate with 95% Confidence Intervals (CI) of Forster’s Tern chicks during each of nine intervals between 10 capture events at Pond A16 in San Francisco Bay during the breeding season.

<p>California Gulls started using Pond A16 heavily on 3 August 2005, and Forster’s Tern chick survival declined significantly thereafter.</p

Chain of events and trophic interactions of Pond A16 concomitant with the low DO and gull intrusion events.

<p>Salinity reduction resulted from management actions during the Initial Stewardship Plan (ISP) of the South Bay Salt Pond Restoration Project in San Francisco Bay.</p

Fine scale water quality and environmental parameters of Pond A16 from 25 July through 25 August 2005.

<p>Water quality conditions from the pond (pale grey line) and weir box (dark grey) datasondes are depicted in the top three panels: Dissolved oxygen (DO; mg L^-1), pH and salinity (PSU), water temperature (°C). Weather conditions are represent in the bottom two panels: air temperature (°C), photosynthetically active radiation (PAR; mol quanta m^-2 s^-2), and wind speed (m s^-1). Data download (circles) and calibration (diamonds) occurred on alternating weeks. Triangles (top two panels) represent ambient South San Francisco Bay water conditions. The DO sensor of both datasondes were fouled and failed to log data from 12–15 August.</p

Species and relative percent collected of 471 individual fish from the Forster’s Tern colony between 20 July and 17 August 2005.

<p>Species and relative percent collected of 471 individual fish from the Forster’s Tern colony between 20 July and 17 August 2005.</p

Model selection table for Forster’s Tern chick daily survival rates.

<p>Model selection table for Forster’s Tern chick daily survival rates.</p

Count (n; solid colors) and dry weight (g; patterns) of fish collected at Forster’s Tern nesting colonies on Pond A16 from 20 July through 17 August 2005.

<p>The five taxa represented (Northern anchovy, NOAN; yellowfin goby, YEGO; longjaw mudsucker, LJMU; unknown goby, UNGO; topsmelt = TOPS), constituted 95% of count and dry weight observed.</p