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Unlike most other microorganisms, zooplankton are technically heterotrophic animals – meaning they cannot produce their own food. Instead, they feed upon phytoplankton like algae, a process responsible for keeping these populations under control. Zooplankton in turn serve as a vital component of the diet of many fish such as Juvenile salmon, striped bass, small splittail and Delta smelt, making them an integral part of both fresh and saltwater ecosystems. 

Dwindling Populations

The gradual warming of the top layer of water has reduced the nutrients available for phytoplankton to photosynthesize. Without this food source, zooplankton populations have plummeted; off the Southern California coast, numbers dropped by 80 percent between 1970 and 1990 after a 1.4 degree Celsius temperature rise in the surface waters. 

The Zooplankton Study

Since 1972, the Zooplankton Study has been monitoring these organisms’ populations throughout eastern San Pablo Bay, Sacramento-San Joaquin Delta and Suisun Marsh. The project is a joint effort between the California Department of Fish and Game, Department of Water Resources (DWR) and the Bureau of Reclamation with the goal of overseeing existing populations and any effect which those artificially-introduced might have on the ecosystem.

This study has observed declines in zooplankton populations. Although interrupted by occasional sporadic increases, on the whole, zooplankton population reduction has persisted.

Zooplankton scarcity has tremendous impacts on the already-endangered Delta smelt, who rely on zooplankton as the primary component of their diet. Additional factors in their habitat have exacerbated the survival struggle of zooplankton, particularly the introduction of overbite clam which feed on them more competitively than Delta smelt.

Toxic algal blooms and dangerous pollutants have also killed significant numbers of zooplankton. As an important staple in the lower levels of the food chain, zooplankton’s population dynamics have far-reaching consequences for the health of aquatic ecosystems.  

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