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Phytoplankton Fuels Delta Food Web

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Phytoplankton Fuels Delta Food Web RESEARCH ARTICLE ▲ ▲ Phytoplankton fuels Delta food web smelt (Hypomesus transpacificus), and commercial species such as chinook Alan D. Jassby salmon (Oncorhynchus tshawytscha) all James E. Cloern show evidence of food limitation dur- Anke B. Müller-Solger ing their first year. Zooplankton, a key ▼ food for young fishes, has also declined (Orsi and Mecum 1996). Some declining zooplankters, especially a mysid shrimp Populations of certain fishes and in- (Neomysis mercedis) and smaller species, also appear to be limited by food supply, vertebrates in the Sacramento-San as does the clam Corbicula fluminea, a Joaquin Delta have declined in abun- dominant benthic invertebrate. dance in recent decades and there is Zooplankton, benthic invertebrates William Sobzcak, former U.S. Geological evidence that food supply is partly Survey postdoctoral researcher, samples for and the larger, more visible fish and responsible. While many sources of zooplankton in the Delta. waterfowl that feed on them form a organic matter in the Delta could be food web that depends ultimately on in- supporting fish populations indi- he Sacramento–San Joaquin River puts of organic matter at its base (fig. 2). In many systems, phytoplankton spe- rectly through the food web (includ- TDelta is a complex mosaic of water- ways that forms the transition zone be- cies play a fundamental role in the or- ing aquatic vegetation and decaying tween San Francisco Bay and its water- ganic matter supply to food webs. organic matter from agricultural shed (fig. 1). Over the last century, the These microscopic plants are respon- drainage), a careful accounting original dominant marsh habitat has sible for primary production, the pho- shows that phytoplankton is the been lost through filling and diking. tosynthetic production of organic dominant food source. Phytoplank- Water flows have changed radically. matter. Other sources of organic matter ton, communities of microscopic Exotic plants and animals have invaded are often present, however, and can free-floating algae, are the most im- or been introduced intentionally, and even be dominant, especially in estuar- toxic contaminants have become wide- ies. In fact, there has been speculation portant food source on a Delta-wide spread (CALFED 2000). The Delta is for many years that organic matter car- scale when both food quantity and now a focus of ecosystem restoration ried in from upstream and from adja- quality are taken into account. These because these changes have been ac- cent terrestrial sources is the main microscopic algae have declined companied by declines in the abun- source sustaining the Delta’s food web. since the late 1960s. Fertilizer and dance of many fish species that use What, then, are the relative roles played pesticide runoff do not appear to the Delta as a migration route, nursery by phytoplankton and other sources in be playing a direct role in long-term or permanent habitat. Some species the organic matter supply to Delta wa- phytoplankton changes; rather, (thicktail chub) have already become terways? The question is a basic one for extinct; others (winter-run chinook restoration of the Delta, because it de- species invasions, increasing water salmon) are now at risk of extinction; termines the focus for increasing food transparency and fluctuations in and still others (splittail, striped bass) supply to declining populations. In ad- water transport are responsible. have dramatically reduced popula- dition to investigating this, we examine Although the potential toxicity of tions. evidence for a long-term decline in the herbicides and pesticides to plank- Several lines of evidence suggest primary food supply and consider the ton in the Delta is well documented, that food limitation has played a role in role of agriculture. these declines. Many fish populations the ecological significance remains Food comes in many forms speculative. Nutrient inputs from ag- in the Delta are declining because of poor survival during the first year of To determine the most important or- ricultural runoff at current levels, in life, which can be caused by food short- ganic matter sources for Delta water- combination with increasing trans- ages (Bennett and Moyle 1996). Sport ways, we combined decades of data parency, could result in harmful al- fish such as striped bass (Morone collected by the California Department gal blooms. saxatilis), native species such as delta of Water Resources (CDWR), U.S. Bu- 104 CALIFORNIA AGRICULTURE, VOLUME 57, NUMBER 4 Glossary reau of Reclamation (USBR) and Algae: Primitive plants usually living in U.S. Geological Survey (USGS), water and occurring as single cells, using a variety of estimation tech- filaments, colonies and irregular aggregations. niques (Jassby and Cloern 2000). Aquatic vascular plant: Higher plants Organic matter sources are usually living in water bodies. May be free- compared in terms of their total organic floating or rooted. Also known as carbon (TOC) content; TOC can come aquatic macrophytes. in both particulate (POC) and dis- Benthic invertebrates: Invertebrates solved (DOC) forms. River input — living on or in the bottom sediments. organic material carried in by the Sac- Benthic microalgae: Microscopic algae ramento, San Joaquin and other rivers living on or just under the surface of the bottom sediments. and creeks — is the largest TOC source overall (fig. 3). Some of this river-borne Biomass: The weight of biological matter, usually measured in terms of material is actually phytoplankton and carbon, dry weight or fresh weight, and phytoplankton-derived detritus trans- expressed on an areal (g m-2) or ported from upstream of the Delta. -3 volumetric (mg m ) basis. Phytoplankton production within Chlorophyll: A green pigment present the Delta and organic matter in agricul- in most plants and essential for the process of photosynthesis, by which tural drainage directly into the Delta these plants obtain most if not all of are next in importance. Agricultural their energy. Chlorophyll a is one form drainage upstream of the Delta is in- that is often assumed an approximate cluded in river inputs and cannot be index of algal biomass. estimated separately with any confi- Detritus: Nonliving particulate and dence. Much of the organic matter in dissolved organic matter. agricultural drainage originates from Metazoa: Multicellular animals, in leaching of DOC from island peat soils. Fig. 1. The Sacramento-San Joaquin River Delta contrast to the more primitive extends upstream from Chipps Island to include protozoa. Discharge from wastewater treatment leveed islands, river channels, sloughs, flooded Phytoplankton: The plant plankton, plants, drainage from tidal marshes islands and tidal marshes. Boxes mark stations consisting mostly of microscopic algae. and production of aquatic vascular used by the California Department of Water Resources to assess water quality, in some Plankton: The community of passively plants such as the submerged Egeria cases since the late 1960s. suspended or only weakly swimming densa are tertiary sources. Urban run- organisms in a body of water, which off, primary production by benthic drift along with the water currents; microalgae and other sources — dominant among external sources (fig. 3). planktonic organisms range in size from tiny plants and animals to large although sometimes important in other Many of these sources are distributed jellyfish, and include the larval stages estuaries — are negligible sources of or- unevenly throughout the Delta, and we of many fishes. ganic matter in the Delta. Phytoplank- can identify areas where aquatic vascu- Primary productivity: The rate at ton is clearly the Delta’s dominant lar plants, tidal marsh drainage or an- which plants incorporate inorganic primary producer, whereas river input other source may dominate at times. carbon into organic matter. Usually measured on a volumetric (mg C m-3 d-1) of organic material from upstream is Nevertheless, phytoplankton produc- or areal (mg C m-2 yr-1) basis. Primary production is often used to refer separately to the amount of organic carbon produced in a particular time interval (mg C m-3 or mg C m-2). Protozoa: Single-celled animals, including amoeba, ciliates and flagellates in aquatic systems. Suspended sediments: Small mineral (clay and silt) particles suspended in waters. Turbidity: The scattering effect that suspended and dissolved solids have on light, imparting a cloudy appearance to water. Primary contributors include suspended sediments, soluble colored organic compounds and microscopic Fig. 2. The metazoan food web can be Fig. 3. Organic matter, measured as total organisms. extremely complex, especially in estuaries, organic carbon (TOC), is produced in Delta Zooplankton: The animal plankton, in and only a small portion of the Delta’s food waterways by photosynthesis and enters from estuaries consisting mostly of protozoa, web is portrayed here. The energy and a variety of external sources. Phytoplankton is rotifers and two crustacean types, the nutrient needs of the metazoan food web the biggest internal source and river transport cladocerans and the copepods. are supported by a variety of organic matter is the biggest external source. Error bars sources. represent standard error among years. http://danr.ucop.edu/calag • OCTOBER-DECEMBER 2003 105 tion, river inputs and agricultural tastes and odors, and contributing dan- phytoplankton production is compa- drainage together account for 90% of gerous substances directly to raw water, rable
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