The following does a great job of explaining the science of how Carp can contribute to the degradation of lakes (Like Delavan Lake) by enhancing the negative effects of settled vegetative nutrients in the lake!
From Joe Pfeiffer and Bonnie Duncan of KCI Associates
Carp are known to vigorously roil the water and bottom substrate in search of food and during spawning seasons. In a comprehensive study completed by Weber and Brown in 2009, “Common carp increased water column nutrients in 75% of the surveyed literature..” (Weber and Brown, Vol. 17 2009, p.526). The research completed in this analysis determined that “Common carp may directly and indirectly increase water column phosphorus, nitrogen, and ammonia as a result of benthic foraging activities, excretion, or destruction and subsequent decomposition of aquatic macrophytes” (Weber and Brown, Vol. 17 2009, p.526). With large numbers of carp spawning and swimming, the effects of large populations can literally alter the physical bottom stratus and water column within a lake, making it difficult for any native fish or vegetative species to thrive over time. Carp activities suspend the sediment and organic material on the bottom of the lake into the water column and cause turbidity or bioturbation resulting in the increased release of nutrients. This bioturbation directly “inhibits [zooplankton] … ingestion of phytoplankton (e.g. Kirk and Gilbert 1990; Kirk 1991), and loss of plant refugia permits them to be readily consumed by planktivores (e.g. Schriver et al. 1995)” (Lougheed, Vanessa, et.al. 1998, p.1190). Bioturbation reduces or eliminates subsurface light needed for plant growth and consequently photosynthetic plant production and oxygen levels dramatically decrease which eliminate macrophytes that provide cover. “Carp also reduce zooplankton and macroinvertebrate populations by predation” (Baldry, 2000 p. 2). Ultimately, fish and wildlife are adversely affected by the loss of zooplankton and macroinvertebrate food sources, and loss of aquatic macrophytes that provide cover for larval and juvenile fish and substrate for eggs and invertebrates (Kahl 1991)” (Baldry, 2000 p. 2).