Darren L. Bade

Effects of wood removal on stream habitat and nitrate uptake in two northeastern US headwater streams

Forested headwater streams play an important role in watershed nutrient dynamics, and wood is thought to be a key factor influencing habitat structure and nitrate-nitrogen dynamics in many forested streams. Because wood in streams can promote nitrogen uptake through denitrification, we hypothesized that nitrate uptake velocities would decrease following wood removal. We measured stream characteristics and nitrate uptake velocities before and after wood manipulation experiments conducted at Hubbard Brook Experimental Forest, NH, and the Sleepers River watershed, VT. The mean size of stream substrates and the amount of riffle habitat increased following wood removal. In contrast to our expectations, summer nitrate uptake velocities increased in the wood removal treatments relative to the reference treatments, possibly because wood removal increased the availability of stable substrates for periphyton growth, therefore increasing nitrate demand in these streams. Our results highlight that effects of wood on stream ecosystems occur through multiple pathways and suggest that the relative importance of these pathways may vary seasonally.

Algal co-limitation by N and P persists after 30 years in Mirror Lake (New Hampshire, USA)

The fundamental question oflimits to ecosystem productivity is a continually important topic in the field oflimnology. Historically, phosphorus (P) has usually been considered the most important limiting nutrient for primary production in freshwater lakes (SCHINDLER 1977). However, the recognition of limitation or co-limitation by other nutrients, in particular nitrogen (N), has generated further interest and understanding about processes that control lake productivity (ELSER et al. 1990). In light of persistent nitrogen pollution in many regions, including the northeastern United States (GALLOWAY & CowLING 2002, ABER et al. 2003), there is even more impetus to examine the potential for nitrogen limitation. Nonpoint sources ofnitrogen pollution are considered to be a key factor in eutrophication of inland waters (CARPENTER et al. 1998). Indeed many examples show lakes becoming less limited by nitrogen, or increased productivity due to nitrogen stimulation (JASSBY et al. 1995, BERGSTRÕM & JANSSON 2006). Several methods were used to asses the nutrient limitation status of Mirror Lake in the early 1970s (GERHART 1975, ÜERHART & LIKENS 1975). In those studies, N and P co-limited phytoplankton primary production. Mirror Lake is located in New Hampshire, United States (LIKENS 1985) in an area known for moderately high deposition o f atmospheric nitrogen (ÜLLINGER et al. 1993 ). Long-term studies o f chemical mass balances conducted from 1980 to 2000 suggested the possibility that the nutrient limitation status of Mirror Lake may have changed; therefore, we repeated one of Gerharts experiments with the intent of discovering whether there had been any change in nutrient limitation in Mirror Lake. Specifically we addressed the question of whether N and P were still co-limiting in Mirror Lake. Surprisingly, we again found co-limitation by N and P after 30 years of continued atmospheric nitrogen pollution.