60 research outputs found
Phytoplankton fuels Delta food web
Populations of certain fishes and invertebrates in the Sacramento-San Joaquin Delta
have declined in abundance in recent decades and there is evidence that food supply
is partly responsible. While many sources of organic matter in the Delta could be
supporting fish populations indirectly through the food web (including aquatic vegetation
and decaying organic matter from agricultural drainage), a careful accounting shows
that phytoplankton is the dominant food source. Phytoplankton, communities of microscopic
free-floating algae, are the most important food source on a Delta-wide scale when
both food quantity and quality are taken into account. These microscopic algae have
declined since the late 1960s. Fertilizer and pesticide runoff do not appear to be
playing a direct role in long-term phytoplankton changes; rather, species invasions,
increasing water transparency and fluctuations in water transport are responsible.
Although the potential toxicity of herbicides and pesticides to plankton in the Delta
is well documented, the ecological significance remains speculative. Nutrient inputs
from agricultural runoff at current levels, in combination with increasing transparency,
could result in harmful algal blooms
Bioavailability of organic matter in a highly disturbed estuary: The role of detrital and algal resources
The importance of algal and detrital food supplies to the planktonic food web of a highly disturbed, estuarine ecosystem was evaluated in response to declining zooplankton and fish populations. We assessed organic matter bioavailability among a diversity of habitats and hydrologic inputs over 2 years in San Francisco Estuary's Sacramento–San Joaquin River Delta. Results show that bioavailable dissolved organic carbon from external riverine sources supports a large component of ecosystem metabolism. However, bioavailable particulate organic carbon derived primarily from internal phytoplankton production is the dominant food supply to the planktonic food web. The relative importance of phytoplankton as a food source is surprising because phytoplankton production is a small component of the ecosystem's organic-matter mass balance. Our results indicate that management plans aimed at modifying the supply of organic matter to riverine, estuarine, and coastal food webs need to incorporate the potentially wide nutritional range represented by different organic matter sources
Untersuchungen zur Morphologie und Elektrophysiologie des Seitenlinienorgans vom Krallenfrosch (Xenopus laevis Daudin)
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