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Open Access Publications from the University of California

Volume 57, Issue 4, 2003

Issue cover


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.

Biomarkers aid understanding of aquatic organism responses to environmental stressors

Biomarkers can be useful tools for understanding the complex interactions that govern organism responses to environmental stressors and their sublethal effects on organism health. We conducted studies on two types of biomarkers: stress proteins and tissue alterations. A study on the freshwater fish medaka demonstrates that the ability to increase cellular stress-protein concentrations at specific life stages can be vitally important for normal embryo development. A field study on Asian clam investigates the usefulness of stress proteins and histopathology as indicators of exposure to and sublethal effects of environmental stressors in the northern San Francisco Bay and Delta.

Landscape changes in Nevada County

Large-scale migration of urban people seeking a better quality of life in rural places has generated considerable concern about “rural sprawl.” In a multimethod, fine-scale, longitudinal study of land ownership and use in Nevada County, we found that this quintessential “exurban” community reveals a complex story of interacting social and ecological change with some reasons for concern, but also optimism. Land-use data from 1957 to 2001 shows dramatic fragmentation of the county’s landscape as a result of increased residential use. The full scale of this transition is not visible because many parcels that are already zoned for further subdivision and residential use remain undeveloped. The related ecological changes have been mixed so far, with tree cover and riparian areas ?recovering from historic mining, ranching and timber harvesting. These changes are not incidental: many residential owners expressed a strong conservation ethic. However, ecologically harmful effects of increased residential use are present as well. In surveys and interviews, rural-residential owners revealed conflicting feelings about their changing landscape: most are concerned about preserving their quality of life and preventing the ecological impacts of further growth, but also do not want additional government regulations. Research on a fine scale into the cultural basis of ecologically beneficial and harmful rural-residential land-use practices can assist policymakers in crafting innovative and effective growth-management institutions.

Olive fruit fly populations measured in Central and Southern California

The olive fruit fly was detected in Southern California in October 1998. This nonnative pest, which can render fruit unmarketable, has since moved throughout California and is now believed to be present wherever olives are grown in the state. Seasonlong trapping of adult flies in the San Joaquin Valley, and Ventura and Santa Barbara counties in 2001 and 2002 showed similarities in seasonal phenology, but also differences primarily due to varying temperatures and fruit availability. In the San Joaquin Valley, fly activity declined as maximum daily temperatures rose above 90?F, but increased when temperatures were between 70?F and 85?F. On the Southern California coast, the combination of available, susceptible fruit and moderate climate throughout the year may allow continuous reproduction of OLF with six or perhaps even seven generations per year.

Insecticide treatments disinfest nursery citrus of glassy-winged sharpshooter

To protect uninfested areas of California from glassy-winged sharpshooter (GWSS), the disinfestation of citrus nursery stock prior to shipment is essential. A nonnative insect, GWSS transmits the bacterium that causes Pierce’s disease in grapevines. In our study, GWSS adults were especially sensitive to two categories of insecticides, the pyrethroids and systemically applied neonicotinoids. Several insecticides, including the carbamate carbaryl and a few of the foliar neonicotinoids were highly effective in preventing GWSS nymphs from successfully emerging from egg masses. While no pesticide treatment will perfectly protect nursery citrus, a treatment plan that includes a combination of insecticides that are effective against adults and emerging nymphs will minimize the chance of transporting GWSS throughout California.

Drip irrigation increases tomato yields in salt-affected soil of San Joaquin Valley

This study evaluated the potential for subsurface drip irrigation in processing tomato to reduce subsurface drainage, control soil salinity and increase farm profits in areas affected by saline, shallow groundwater. ?Subsurface drip irrigation systems were installed in three fields of fine-textured, salt-affected soil along the West Side of the San Joaquin Valley. No subsurface drainage systems were installed in these fields. Yield and quality of processing tomato were compared with sprinkler irrigation systems. Yields increased 5.4 tons per acre to 10.1 tons per acre in the drip systems with similar amounts of applied water. The solids content of drip-irrigated processing tomato was acceptable. Water-table levels showed that properly managed drip systems could reduce percolation below the root zone, reducing subsurface drainage. Yields of the drip systems were also similar over a range of soil salinity levels.

Can almond trees directly dictate their irrigation needs?

Many of California’s fruit and nut growers have already embraced more efficient irrigation systems (drip and microsprinkler) and adopted scientific irrigation-scheduling methods that closely match net applied water to evapotranspiration (ETc). Further improvements in irrigation efficiency may be possible by using regulated deficit irrigation (RDI) to purposely stress trees at specific times of the season. Tree-based RDI triggers for irrigation scheduling, such as stem water potential (SWP) and maximum daily trunk shrinkage (MDS), as opposed to soil and atmospheric measurements, have the major advantage of being directly linked to crop productivity. The current state of the art in plant-based scheduling is SWP and MDS, but adoption is hampered by the lack of field studies validating its effectiveness. We conducted an experiment in a commercial almond orchard to evaluate the suitability of MDS measurements as indicators for RDI management. Small, electronic sensors affixed to the tree trunks continuously recorded diameters from which MDS values were gleaned and used to schedule two RDI treatments. We found that with the less severe RDI regime, less water was applied relative to the cooperator’s nearly fully irrigated trees with no significant reduction in kernel size or other important almond parameters. In fact, the RDI regimes accelerated hull-split, decreased kernel water content and increased the nut-kernel percentage at harvest — all desirable almond results. We have demonstrated, for the first time in California, that RDI can be successfully scheduled based entirely on continuously recorded, tree-based electronic data. We believe that MDS measurements have some operational advantages over SWP, including lower labor costs and the ability to be directly incorporated into remotely operated, electronic controllers.