Phytoplankton fuels Delta food web

T Sacramento–San Joaquin River Delta is a complex mosaic of waterways that forms the transition zone between San Francisco Bay and its watershed (fig. 1). Over the last century, the original dominant marsh habitat has been lost through filling and diking. Water flows have changed radically. Exotic plants and animals have invaded or been introduced intentionally, and toxic contaminants have become widespread (CALFED 2000). The Delta is now a focus of ecosystem restoration because these changes have been accompanied by declines in the abundance of many fish species that use the Delta as a migration route, nursery or permanent habitat. Some species (thicktail chub) have already become extinct; others (winter-run chinook salmon) are now at risk of extinction; and still others (splittail, striped bass) have dramatically reduced populations. Several lines of evidence suggest that food limitation has played a role in these declines. Many fish populations in the Delta are declining because of poor survival during the first year of life, which can be caused by food shortages (Bennett and Moyle 1996). Sport fish such as striped bass (Morone saxatilis), native species such as delta smelt (Hypomesus transpacificus), and commercial species such as chinook salmon (Oncorhynchus tshawytscha) all show evidence of food limitation during their first year. Zooplankton, a key food for young fishes, has also declined (Orsi and Mecum 1996). Some declining zooplankters, especially a mysid shrimp (Neomysis mercedis) and smaller species, also appear to be limited by food supply, as does the clam Corbicula fluminea, a dominant benthic invertebrate. Zooplankton, benthic invertebrates and the larger, more visible fish and waterfowl that feed on them form a food web that depends ultimately on inputs of organic matter at its base (fig. 2). In many systems, phytoplankton species play a fundamental role in the organic matter supply to food webs. These microscopic plants are responsible for primary production, the photosynthetic production of organic matter. Other sources of organic matter are often present, however, and can even be dominant, especially in estuaries. In fact, there has been speculation for many years that organic matter carried in from upstream and from adjacent terrestrial sources is the main source sustaining the Delta’s food web. What, then, are the relative roles played by phytoplankton and other sources in the organic matter supply to Delta waterways? The question is a basic one for restoration of the Delta, because it determines the focus for increasing food supply to declining populations. In addition to investigating this, we examine evidence for a long-term decline in the primary food supply and consider the role of agriculture.