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Maintaining Water Quality while Restoring South Bay Salt Ponds to Wetlands Robert Schlipf [[email protected]], Andree Greenberg, Brian Wines, Shin-Roei Lee, Wil Bruhns Post Phase I 3.0 10,000 2.0 1.0 5,000 0.0 5/5 6/4 7/4 8/3 9/2 10/2 11/1 Tidal Marsh 4.0 2007 0 Problem The operation of former salt ponds as managed ponds (i.e., circulating Bay waters through water quality control structures) has threatened water quality especially by decreasing dissolved oxygen (DO). This is because former salt ponds have large surface areas, shallow water depths, and long residence times which makes them extremely conducive to excessive algal growth, which can lead to a severe depletion in DO levels when algae suffer die-off or respire during nighttime hours. Shallow open water habitat creates excellent foraging opportunities for shorebirds The DO levels in former salt ponds are not protective of water quality since they are often below those considered safe for fish survival as shown in the graph below from the U.S. EPA’s Ambient Water Quality Criteria for Dissolved Oxygen*, which indicates that most fish will survive exposure above 3 mg/L, but continuous exposure below this threshold will result in mortality in many species (*EPA Publication Number 440/5-86-003). Dead topsmelt in Pond A7 from low dissolved oxygen levels Methods To improve water quality in former salt ponds, the following methods have been used to address low DO levels: 1. Switch pond circulation patterns from directional to muted tidal 2. Implement discharge timing 3. Install baffles 4. Install solar powered aerators All of the above corrective measures have had marginal impacts on improving DO levels. This is because these corrective measures do not address the root of the problem, which is pond design. 1,880 9,674 3,317 1,40 0 709 Habitat Type Restoring salt ponds to tidal marsh will provide the most improvement to water quality because marsh systems that are tidally connected to the estuary provide: ● improved water quality by filtering and fixing pollutants ● nursery habitat and protection from predation for juvenile fish ● significant biological productivity to the estuarine system ● habitat for rare, endangered, and other species Mature tidal marsh Large surface areas and shallow water make former salt ponds extremely conducive to excessive algal growth Dead leopard shark in Pond A7 from low dissolved oxygen levels 13,220 Reversible Muted Tidal 15,000 5.0 Reconfigured Managed Ponds Phase I - Habitat Modifications Alviso Ponds, Weekly 10th Percentile of All Ponds 6.0 Salt Ponds * From 1978 to 1996, the Aquatic Species Program evaluated the potential to develop renewable transportation fuels from algae. The main focus of this program was to develop biodiesel from algae grown in ponds. Implications Water quality is severely impacted when former salt ponds are operated as managed ponds because excessive algae growth leads to low DO levels. To protect water quality, ponds managed for bird habitat should be reconfigured to improve water circulation patterns. Reconfiguration of two ponds systems will occur in Phase I of the SBSPRP. The graph below shows Phase I actions that will begin within the next five years. Tidal Marsh One of the challenges with this restoration project is that former salt ponds provide excellent habitat for birds. Unfortunately, the conditions that make ponds suitable for birds (i.e., shallow open water habitat over large surface areas) are the same as those that are conducive to excessive algal growth. In fact, in order to grow algae, the U.S. Department of Energy’s Aquatic Species Program* recommends constructing ponds with large surface areas and shallow water depths because of the need to keep algae exposed to sunlight. In other words, former salt ponds greatly resemble the U.S. Department of Energy’s recommendations for designing an algae farm. Data/Results To measure the effect of pond discharges on receiving waters, DO is measured continuously at pond discharge points. These data show that DO levels are depleted especially in the late summer months when algae proliferate in former salt ponds and the days get longer and hotter. The graph below shows the average of weekly 10th percentiles in the Alviso Ponds for 2007. As expected, DO levels trend lower in the later summer months (August and September) before recovering in the fall. Salt Ponds Bird Habitat vs Algae Farms Acres The purpose of the South Bay Salt Pond Restoration Project (SBSPRP) is to restore about 13,000 acres of salt ponds to a mix of habitats, in particular, tidal marsh, while providing flood protection and recreational opportunities. The project area is shown below: Dissolved Oxygen (mg/L) Project Purpose San Francisco Bay Regional Water Quality Control Board, Oakland, CA The effect of low DO in former salt ponds is not limited to fish mortality. Diurnal shifts in DO levels caused by excessive algal growth will also have sub-lethal effects on fish. These effects are documented in U.S. EPA’s DO Report, which cites a study where DO levels were kept at 3 mg/L for 8 hours, and 8.3 mg/L for the remaining hours over 9 days. This diurnal shift in DO levels produced significant stress on 2 of the 3 fish species tested. During periods of low DO, fish lost their natural color, increased their ventilation rate, ignored food, and often vomited food that had been consumed under the higher oxygen environment. This study suggests that values around 3 mg/L over a sizable period of the day will harm aquatic life. Conclusions •The design of former salt ponds makes them conducive to excessive algal growth. If former salt ponds are operated as managed ponds to provide bird habitat, the configuration of these ponds should be altered. •To maximize water quality, former salt ponds should be restored to tidal marsh within the constraints of meeting habitat goals for birds and protecting human life and property in the South Bay. •This very large restoration project has the potential to benefit humans, water quality, and biological species and should be phased over the next decades to assure that problems are minimized and benefits are maximized.