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Page 2 of 3
 Daphnia, commonly called a water flea, is a filter feeding planktonic crustacean about 2 mm long. It suddenly became an important member of the zooplankton in Lake Washington in 1976, although it had been present in small numbers previously. Daphnia is an efficient filter feeder and can reduce algae populations quickly, thus increasing water transparency. While Daphnia can consume some kinds of filamentous plankton, Oscillatoria clogs its filtering apparatus so it is unable to feed. Thus, the increase in Daphnia coincided with the demise of Oscillatoria in the lake. Since Daphnia can reproduce quickly to exploit favorable conditions, its numbers can fluctuate dramatically through the year, although peak abundances occur in May and June when temperature and sunlight trigger increased activity.
Besides the decrease of Oscillatoria and the increase in Daphnia, there was a reduction in the population of possum shrimp. Neomysis mercedis, the possum shrimp, is a planktonic crustacean that can reach a total length of about 14 mm (0.5 inch). It has been shown to have a strong feeding preference for Daphnia and is the main predator on Daphnia. In the late 1970s, Paul Murtaugh of the University of Washington studied the gut contents of Neomysis and concluded that it is an especially potent predator on Daphnia, capable of strongly influencing Daphnia abundances. Neomysis is a native species that has been present in Lake Washington for many decades, but has been scarce since 1968. Coinciding with its decline has been a rise in the number of long-fin smelt, which were discovered in the lake in 1960 by Robert Dryfoos, a UW student at the time.
Lake Washington’s longfin smelt is one of two landlocked populations of this small anadromous fish; the other population is in Harrison Lake, British Columbia. The species is distributed on the Pacific coast from northern California to northern British Columbia. Studies from the UW and Washington Department of Fisheries have shown the longfin smelt to be highly selective in feeding on Neomysis, with some two-year olds having a diet composed of 96% Neomysis. Younger longfin smelt are less specialized, eating Neomysis, Daphnia, and other crustacean zooplankton. Reasons for the increase in these smelt are not obvious, but the increase may be linked to inadvertent improvements in breeding habitat in the Cedar River; where the vast majority of spawning takes place. Government agencies have been working to sustain salmon habitat and to control flood damage in the area, and the smelt may have benefited from these habitat improvements.
Now we can see how changes in the amount of phosphorus, mainly from sewage outfalls into the lake, affected the species composition of the lake. The cyanobacteria Oscillatoria were not able to thrive in the lake after sewage diversions decreased the level of phosphorus input, so the numbers of Daphnia increased. The Daphnia populations also increased because its main predator, Neomysis, was reduced by the longfin smelt. Filter feeding by Daphnia helped reduce the green algae populations, so water clarity and quality increased.
 Sockeye salmon are another species whose numbers increased during the lake's period of eutrophication, although the increase was probably not directly related to the level of phosphorus in the lake. Sockeye salmon are unique among salmon in that the smolts have a yearlong phase in freshwater lakes before to their migration to the sea. Sockeye had been planted in the Cedar River in 1953 but were in relatively low numbers until the mid 1960s. In 1970 the fish were numerous enough for the state to permit commercial fishing in the lake. The increase in sockeye may be due to inadvertent benefits reaped to the spawning beds, when flood control measures and a halt to channel dredging (because of equipment failure) just happened to reduce the silt accumulation on the gravel spawning beds. The smolts from Lake Washington are the largest of their species, but numbers have been down in recent years. Research into the cause of the decline is under way by several agencies. It includes research on food supply, predation, and physical damage from the Government Locks during out-migration.
Contrary to common notion, UW research has not shown northern squawfish to be preying substantially on sockeye in Lake Washington, but cutthroat and rainbow trout have actually been implicated as being predators on the sockeye. Northern squawfish, however, are certainly adaptable in their diet, readily able to shift to different prey items. Largemouth and smallmouth bass are potential predators on sockeye, and one theory is that an increase in the number of boat docks has resulted in an increase in habitat for the bass. However, the spatial overlap between them and the sockeye may not be sufficient for there to be much of an impact. The major food supply for sockeye fry in winter is unknown. They do feed on Daphnia, but the sockeye fry appear in the lake about March or April, a month or two before Daphnia becomes abundant.
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