San Francisco Bay has been experiencing significant increases in phytoplankton biomass since the late 1990s, a fact supported by a U.S. Geological Survey analysis of water quality between 1978 and 2009 showing that water column chlorophyll a has increased 30–50% (from Suisun to South Bay). In addition, DO concentrations have declined (1.6% to 2.5% per decade in South Bay and Suisun Bay, respectively). There also is evidence that the historic resilience of San Francisco Bay to the harmful effects of nutrient enrichment is weakening.
The study points out the shortcomings of using ambient nutrient concentration criteria to prevent eutrophication. For example, it is generally not effective for assessing eutrophication and subsequent impact on designated uses after ecological changes already have taken place. Also, biological responses to nutrients depend on a variety of mitigating factors such as morphology and substrate characteristics, tidal energy, stratification, temperature, light availability, and biological community structure. Therefore, high concentrations are not necessarily a good indicator of eutrophication and low concentrations do not necessarily indicate the absence of eutrophication.
The California State Water Resources Control Board (SWRCB) is developing nutrient water quality objectives for the state’s surface waters using the Nutrient Numeric Endpoint (NNE) assessment framework. The term was coined by the SWRCB to describe the strategy for developing nutrient objectives for California. The NNE assessment framework establishes a suite of numeric endpoints based on the ecological response of an aquatic water body to nutrient overenrichment [eutrophication (e.g. algal biomass, DO)]. In lieu of understanding the effects of nutrients on estuaries, the authors used a conceptual model to look at causes and cofactors (e.g., hydrologic residence times, mixing characteristics, water temperature, light climate, grazing pressure, and coastal upwelling). Increased nutrient inputs and alterations in cofactors can result in changes to aquatic primary producers, altered water and sediment biogeochemisty, and altered community structure of secondary and tertiary consumers (i.e., invertebrates and fish, birds, and mammals, respectively). See the figure below.
From McKee et al. (2011)
The NNE assessment framework is a structured set of decision rules that helps to classify a water body in categories from minimally to very disturbed either to determine if the water body is meeting beneficial use, or to establish total maximum daily load numeric targets. Developing an assessment framework begins by choosing response indicators, which were reviewed using four criteria: (1) strong linkage to beneficial uses; (2) well-vetted means of measurement; (3) ability to model the relationship between the indicator, nutrient loads, and other management controls; and (4) has an acceptable signal-to-noise ratio to assess eutrophication.
Indicators varied among four habitat types: (1) unvegetated subtidal, (2) seagrass and brackish submerged aquatic vegetation (SAV), (3) intertidal flats, and (4) tidally muted habitats (e.g., estuarine diked Baylands). Two types of indicators were designated: primary indicators, which met all evaluation criteria and would, therefore, be expected to be a primary line of evidence of the NNE assessment framework for SF Bay; and supporting indicators, which fell short of meeting evaluation criteria, but might be used as supporting lines of evidence. This terminology is used to provide a level of confidence in how the indicators should be employed in a context of multiple lines of evidence. Candidate indicators reviewed in this report for potential development within the NNE assessment framework included phytoplankton, macroalgae, seagrass and brackish water SAV, benthic macroinvertebrates, jellyfish, nutrient concentrations and ratios, and DO.
The review found four types of indicators met all evaluation criteria and designated them as primary indicators:
- DO (for all subtidal habitat)
- Phytoplankton biomass, productivity, and assemblage (for all subtidal habitats)
- Cyanobacterial abundance and toxin concentration (for all subtidal habitats)
- Macroalgal biomass and cover (for intertidal habitats, tidally muted habitats, and seagrass habitats)
Other indicators evaluated met up to three of the review criteria and were designated as supporting indicators: harmful algal bloom cell counts and toxin concentration, urea and ammonium (all subtidal habitats), light attenuation, and epiphyte load (seagrass/brackish SAV habitats). See the table below for a summary of review of candidate NNE indicators for San Francisco Bay (McKee et al. 2011).
R
Reference:
McKee, L., M. Sutula, A. Gilbreath, J. Beagle, D. Gluchowski, and J. Hunt. 2011. Numeric Nutrient Endpoint Development for San Francisco Bay Estuary: Literature Review and Data Gaps Analysis. Southern California Coastal Water Research Project Technical Report 644. Accessed October 2016. http://www.sfei.org/sites/default/files/biblio_files/644_SFBayNNE_LitReview_Final.pdf Exit.