Phytoplankton Regulation in a Eutrophic Tidal River (San Joaquin River, California)

Abstract

As in many U.S. estuaries, the tidal San Joaquin River exhibits elevated organic matter production that interferes with beneficial uses of the river, including fish spawning and migration. High phytoplankton biomass in the tidal river is consequently a focus of management strategies. An unusually long and comprehensive monitoring dataset enabled identification of the determinants of phytoplankton biomass. Phytoplankton carrying capacity may be set by nitrogen or phosphorus during extreme drought years but, in most years, growth rate is light-limited. The size of the annual phytoplankton bloom depends primarily on river discharge during late spring and early summer, which determines the cumulative light exposure in transit downstream. The biomass-discharge relationship has shifted over the years, for reasons as yet unknown. Water diversions from the tidal San Joaquin River also affect residence time during passage downstream and may have resulted in more than a doubling of peak concentration in some years. Dam construction and accompanying changes in storageand-release patterns from upstream reservoirs have caused a long-term decrease in the frequency of large blooms since the early 1980s, but projected climate change favors a future increase. Only large decreases in nonpoint nutrient sources will limit phytoplankton biomass reliably. Growth rate and concentration could increase if nonpoint source management decreases mineral suspensoid load but does not decrease nutrient load sufficiently. Small changes in water storage and release patterns due to dam operation have a major influence on peak phytoplankton biomass, and offer a near-term approach for management of nuisance algal blooms.

Factors Affecting Fish Entrainment into Massive Water Diversions in a Tidal Freshwater Estuary: Can Fish Losses be Managed?

“…Estuaries are particularly sensitive to water diversions because reduced freshwater inflows can alter sediment budgets (Wright and Schoellhamer 2005), water quality (Lane et al 1999;Monsen et al 2007), biological productivity (Jassby and Cloern 2000;Jassby 2005), and distribution of invertebrates (Stora and Arnoux 1983;Rodriguez et al 2001;Kimmerer 2002a;Massengill 2004) and fishes (Kimmerer 2002a;Feyrer et al 2007). Natural mortality for young fishes is very high (Houde 1987); entrainment adds additional mortality that can compromise population resilience (Barnthouse et al 1983;Stevens et al 1985;Boreman and Goodyear 1988;Pawson and Eaton 1999;Bennett 2005;Kimmerer 2008).…”

Low Dissolved Oxygen in an Estuarine Channel (San Joaquin River, California): Mechanisms and Models Based on Long-Term Time Series

“…Delta inflows comprise, on average, 84% Sacramento River flow, 13% San Joaquin River flow, and 3% from smaller rivers. The San Joaquin River extends from the Delta upstream past the city of Fresno, draining a watershed area of about 19,000 km 2 (Figure 1 in Jassby 2005). Its river valley is a major center of agricultural production.…”

“…The proportion of flow down Old River depends on water exports and the presence or absence of a temporary rock barrier-the Head of Old River barrier-constructed annually at the confluence of the Old and San Joaquin rivers to protect outmigrating juvenile salmon from the federal and state pumping plants. The monthly average of measured net discharge values can be predicted using river discharge at Vernalis, water exports, and the state of the barrier, allowing construction of a surrogate net discharge series (Q net , m 3 s -1 ) for the entire period of interest (Jassby 2005).…”

“…Cloern and others (1995) developed an empirical expression for this ratio dependent on temperature, mean water column irradiance, and nutrient concentration, which was used to estimate the organic carbon equivalent of phytoplankton chlorophyll a. The details are described by Jassby (2005). The carbonaceous BOD for a typical phytoplankton cell is 2.69 g O (g C) -1 , and the nitrogenous BOD due to nitrification is 0.804 g O (g C) -1 , an additional 30% (Chapra 1997).…”

“…Several government agencies have maintained monitoring programs for decades, mostly for determining compliance with water quality objectives. Recently, we used these data to investigate regulation of phytoplankton concentrations in the tidal San Joaquin River upstream of the Ship Channel (Jassby 2005). Our purpose here is to determine what this extensive historical monitoring dataset going back to the 1960s can tell us about the nature of hypoxia in the Ship Channel and the mechanisms underlying it.…”