Historical and Recent Breeding of the Western Grebe and Clark's Grebe in a Severely Impaired Ecosystem at Clear Lake, California

Abstract

Clear Lake is a large (176.7-km 2 ) natural lake in northern California, USA. An estimated 1000 pairs of Western Grebes (Aechmophorus occidentalis) and Clark's Grebes (A. clarkii) nested on the lake prior to massive applications of the pesticide dichlorodiphenyldichloroethane (DDD) to kill the Clear Lake gnat (Chaoborus astictopus) during 1949-1957. Biomagnification of DDD was implicated as the cause of several mass mortality events of grebes and short-term reproductive failure in the 1950s, although other factors such as mercury (Hg) poisoning may have contributed. Successful nesting resumed in 1960 and increased gradually, with up to 165 nests in 1967 but fewer through 1976. Subsequent surveys revealed up to 390 nests annually during 1992-1995 and up to 2675 nests annually during 1996-2010. We conducted 7-29 nest surveys annually during 2010-2019 and found a mean of 3123 nests per year. The annual number of nests varied dramatically (range 898 to 5936) and was unrelated to water level. The grebes nested at 28 sites in Clear Lake and 9 sites in associated wetlands, with proportionately more nests on Clear Lake and fewer nests in adjacent wetlands during years with low water level. Western Grebes attended 84.9% of the nests and Clark's Grebes attended 15.1%. During years with low water, there were proportionately more open water nests, which were anchored to submergent vegetation farther from shore and in deeper water than marsh nests, which were more densely placed in emergent vegetation closer to shore in shallower water. Brood surveys along the perimeter of the lake at the end of each breeding season revealed considerable variation in the number of young (1-599, x -= 153.9), young per nest (0.00025-0.118, x -= 0.04), and productivity ratio of young to adults (0.0006-0.252, x -= 0.07). Reproductive success was unrelated to water level. Wind-generated waves were a major cause of nest failure, destroying up to 35% of a year's nests. Predation on eggs was also a major cause of nest failure. American Crow (Corvus brachyrhynchos) was the dominant predator during the day (84% of predation events), and northern raccoon (Procyon lotor) was the dominant predator at night (98% of predation events). Nocturnal predation by mammals occurred most frequently when water level was high because nests were placed closer to shore and were more accessible. The grebes' breeding population has recovered since DDD biomagnification during the 1950s and may be larger due to 2 introduced fish species competing for plankton with benthic larvae of the Clear Lake gnat, which previously transferred large quantities of biomass from aquatic to terrestrial ecosystems during emergence. However, the grebes' reproductive success remains lower at Clear Lake than at several other breeding localities. The bioaccumulation and biomagnification of toxic chemicals and the periodic reductions of prey fish from hypoxic areas of the lake during periods of intense cyanobacterial and algal blooms may reduce reproductive success.

Identification of Impacts and Human Health Risks Produced by the Presence of Pesticides in the Environment I. Pesticides Behaviour in the Environment

“…The concentration of DDT measured in the body of two dolphins from the Black Sea was 50-100 times higher than the values recorded in the body of cetaceans from the North Sea, the Atlantic Ocean and the Baltic Sea [40,41]. A similar study was carried out on Clear Lake in California, located about 150 km from the city of San Francisco, where diluted solutions of DDD (pesticides from the same chemical group as DDT) were applied in order to combat mosquitoes, which created discomfort in the surroundings of the lake [42]. After about 3-4 years of repeated use, the mosquitoes developed resistance, and at the same time strange effects began to appear, the most serious being the massive death, almost to the point of extinction, of some water birds spread in the area (Western grebes) and reduction of the fish population.…”