2007: Crayfish N:P in Disturbed and Referenced Streams

Lindsey North (Biology) College Of William and Mary

The Williamsburg and James City County areas are becoming increasingly developed, leading to a higher amount of impervious surface. With so much area impermeable to water, stormwater and nutrient runoff can flow much faster over land directly to streams. To help slow this process, there have been over 500 retention ponds installed. Retention ponds are an attempt to trap runoff and release it at a slower rate into streams. A large amount of nutrients flow into these ponds and is taken up by the pond algae. Algae is released along with excess water during storm events into a stream attached to the pond. The algae becomes part of the periphyton of the stream and, therefore, a food source to grazers such as crayfish.

Crayfish can be used as indicators of stream health and retention pond function since elemental ratios of the water are reflected in crayfish stoichiometry. For this reason, I decided to research whether there is a difference in crayfish nitrogen to phosphorus ratios between reference streams and disturbed streams. A reference stream was defined as one which was secluded from development and did not receive outflow from a retention pond. A disturbed stream was defined as one in a developed area receiving outflow from a retention pond. I used 7 disturbed sites and 8 reference sites for my analyses. Since crayfish eat more plant matter than animal matter, I hypothesized there would be a significant difference in crayfish N:P between disturbed and reference sites, with disturbed sites having higher N:P ratios due to the higher level of N in the retention pond. To sample the crayfish I used three methods: dip netting, baited minnow pots with chicken liver and searching by hand under logs and rocks. Lab analysis was done only on the freeze-dried tail muscle of each crayfish, using an elemental analyzer for nitrogen levels and spectrophotometer for measuring phosphorus levels.

Very high levels of nitrogen were observed in each crayfish, in both disturbed and reference sites, with a mean of 13.29. The variation in N:P came primarily from the phosphorus levels. There was no significant difference between originally designated disturbed and reference sites (p=0.826), but when York River State Park was made its own category a significant difference was observed. Between all groups there was a significant difference (p=0.000) and between the new disturbed and reference groups there was also a significant difference (p=0.003). More research is needed in regard to nitrogen levels and the conditions at York River State Park.

For additional documentation Lindsey North provided a PowerPoint Presentation entitled "Crayfish N:P in Disturbed and Referenced Streams"  provided here in PDF form.