Abstract
doi: https://doi.org/10.15447/sfews.2016v14iss4art4 Increasing clarity of Delta waters, the emergence of harmful algal blooms, the proliferation of aquatic water weeds, and the altered food web of the Delta have brought nutrient dynamics to the forefront. This paper focuses on the sources of nutrients, the transformation and uptake of nutrients, and the links of nutrients to primary producers. The largest loads of nutrients to the Delta come from the Sacramento River with the San Joaquin River seasonally important, especially in the summer. Nutrient concentrations reflect riverine inputs in winter and internal biological processes during periods of lower flow with internal nitrogen losses within the Delta estimated at approximately 30% annually. Light regime, grazing pressure, and nutrient availability influence rates of primary production at different times and locations within the Delta. The roles of the chemical form of dissolved inorganic nitrogen in growth rates of primary producers in the Delta and the structure of the open-water algal community are currently topics of much interest and considerable debate. Harmful algal blooms have been noted since the late 1990s, and the extent of invasive aquatic macrophytes (both submerged and free-floating forms) has increased especially during years of drought. Elevated nutrient loads must be considered in terms of their ability to support this excess biomass. Modern sensor technology and networks are now deployed that make high-frequency measurements of nitrate, ammonium, and phosphate. Data from such instruments allow a much more detailed assessment of the spatial and temporal dynamics of nutrients. Four fruitful directions for future research include utilizing continuous sensor data to estimate rates of primary production and ecosystem respiration, linking hydrodynamic models of the Delta with the transport and fate of dissolved nutrients, studying nutrient dynamics in various habitat types, and exploring the use of stable isotopes to trace the movement and fate of effluent-derived nutrients.
Highlights
The paradigm concerning nutrients in California’s Sacramento–San Joaquin River Delta (Delta), developed over several decades of research, is that nutrient levels are high, nutrient limitation is rare, and factors other than nutrients regulate the rates of primary production (Jassby et al 2002)
total phosphorus (TP) loading to the Delta was investigated using the same modeling approach (Domagalski and Saleh 2015), and this study suggested that agricultural lands contributed 65% and 58% of the total loading from the Sacramento River and San Joaquin River drainages, respectively
Similar to observations made for primary production, phytoplankton nitrogen uptake rates declined in the middle reaches of the Sacramento River between Hood and Rio Vista, and phytoplankton relied on ammonium as their primary inorganic nitrogen source
Summary
The paradigm concerning nutrients in California’s Sacramento–San Joaquin River Delta (Delta), developed over several decades of research, is that nutrient levels are high, nutrient limitation is rare, and factors other than nutrients regulate the rates of primary production (Jassby et al 2002). Despite generally abundant nutrients, there has been persistently low phytoplankton biomass throughout the northern estuary and Delta (Cloern 1996), and measurements of primary production in Suisun Bay (Kimmerer et al 2012; Parker et al 2012c; Wilkerson et al 2015) are among the lowest of estuarine–coastal ecosystems in the world (Cloern et al 2014).
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