Kristin E. Judd

VARIATION IN DISSOLVED ORGANIC MATTER CONTROLS BACTERIAL PRODUCTION AND COMMUNITY COMPOSITION

An ongoing debate in ecology revolves around how species composition and ecosystem function are related. To address the mechanistic controls of this relationship, we manipulated the composition of dissolved organic matter (DOM) fed to aquatic bacteria to determine effects on both bacterial activity and community composition. Sites along terrestrial to aquatic flow paths were chosen to simulate movement of DOM through catchments, and DOM was fed to downslope and control bacterial communities. Bacterial production was measured, and DOM chemistry and bacterial community composition (using denaturing gradient gel electrophoresis of 16S rRNA genes) were characterized following incubations. Bacterial production, dissolved organic carbon (DOC)-specific bacterial production, and DOC consumption were greatest in mesocosms fed soil water DOM; soil water DOM enhanced lake and stream bacterial production by 320-670% relative to lake and stream controls. Stream DOM added to lake bacteria depressed bacterial production relative to lake controls in the early season (-78%) but not the mid-season experiment. Addition of upslope DOM to stream and lake bacterial communities resulted in significant changes in bacterial community composition relative to controls. In four of five DOM treatments, the bacterial community composition converged to the DOM source community regardless of the initial inoculum. These results demonstrate that shifts in the supply of natural DOM were followed by changes in both bacterial production and community composition, suggesting that changes in function are likely predicated on at least an initial change in the community composition. The results indicate that variation in DOM composition of soil and surface waters influences bacterial community dynamics and controls rates of carbon processing in set patterns across the landscape.

Production and export of dissolved C in arctic tundra mesocosms: the roles of vegetation and water flow

To better understand carbon (C) cycling in arctic tundra we measureddissolved C production and export rates in mesocosms of three tundra vegetationtypes: tussock, inter-tussock and wet sedge. Three flushing frequencies wereused to simulate storm events and determine potential mass export of dissolved Cunder increased soil water flow scenarios. Dissolved C production and exportrates differed between vegetation types (inter-tussock < tussock < wetsedge). In the absence of flushing, dissolved organic C (DOC) dominatedproduction in tussock and inter-tussock soils but was consumed in wet sedgesoils (8.3, 32.7, and −0.4 μg C g soil−1day−1). Soil water dissolved C concentrations declined over time when flushedat high and medium frequencies but were variable at low flushing frequency.Total yield of dissolved C and DOC increased with increased flushing frequency.The ratio of DOC to dissolved inorganic C exported dropped with increasedflushing under tussock but not inter-tussock or wet sedge vegetation. Massexport per liter of water added declined as flushing frequency increased intussock and inter-tussock mesocosms. Export and production of dissolved C werestrongly correlated with above ground biomass, but not with photosynthetic ratesor below ground biomass. DOC quality was examined by measuring production ofToolik Lake bacteria fed mesocosm soil water. When normalized for DOCconcentration, wet sedge soil water supported significantly higher bacterialproduction. Our results indicate that arctic tundra soils have high potentialsfor dissolved C export, that water flow and vegetation type mainly controldissolved C export, and that responses of aquatic microbes to terrestrial inputsdepend on the vegetation type in the watershed.

Bacterial responses in activity and community composition to photo-oxidation of dissolved organic matter from soil and surface waters

Abstract.To better understand how photo-oxidation of dissolved organic matter (DOM) affects microbial activity and DOM processing along hydrological flow paths in an arctic catchment, we conducted experiments measuring responses in bacterial production (BP) to sunlight-exposed DOM from up-slope sources, including soil, stream, and lake water. Overall, sunlight exposure significantly reduced specific ultra-violet absorbance and dissolved organic carbon concentrations and increased phenolic concentrations; these changes were more pronounced in soil than in surface waters. Sunlight-exposed DOM reduced BP in common inoculum bioassay experiments at short time scales (hours), but the magnitude of the effect differed among DOM sources and was related to phenolic content; effects were more negative on DOM sources with high phenolic contents. In contrast, longer contact times (weeks) between bacteria and photo-oxidized soil, stream, and lake water DOM enhanced, did not change, and reduced BP, respectively. In these longer-term experiments, sunlight-exposed DOM had a greater positive effect on lake than stream BP, and caused shifts in bacterial community composition (based on denaturing gradient gel electrophoresis of bacterial-specific 16S rDNA). Changes in bacterial community composition were greater in response to sunlight-exposed DOM from soils than from the stream. Our findings suggest that overall impacts of DOM photo-oxidation in aquatic environments depends on DOM source, and that contrasting effects of photo-oxidized DOM at varying time scales may be partly due to shifts in the bacterial community composition to groups better able to consume photoproducts or tolerate harmful radicals.

A Case History: Effects of Mixing Regime on Nutrient Dynamics and Community Structure in Third Sister Lake, Michigan During Late Winter and Early Spring 2003

Abstract We studied the winter to spring transition in Third Sister Lake (TSL), a small glacial lake in southeastern Michigan, to determine the effect of mixing regime on nutrient dynamics and community structure in an urban temperate lake. After ice-off, the oxycline was mixed downward from 3 to 6 m depth, resulting in addition of 5 mg m−2 P-SRP, 857 mg m−2 N-NH4 +, and 400 mg m−2 N-NO3 − to the epilimnion, but trapping 299 mg m−2 P-SRP, 7877 mg m−2 N-NH4 + and 36 mg m−2 N-NO3 − in the bottom waters. Nutrients supplied by snow melt runoff (138 mg m−2 P-SRP, 430 mg m−2 N-NH4 +, 596 mg m−2 N-NO3 −) were an order of magnitude greater than rain event loads (0.13 mg m−2 P-SRP, 0.17 mg m−2 N-NH4 +, and 1.05 mg m−2 N-NO3 −) during the transition time from ice cover to open water. Reduced spring mixing did not have a large impact on N:P molar ratios, because external N:P ratios were low (7.5) compensating for reduced supply of P from the bottom waters. Bacterial production was greater in the hypolimnion than in the epilimnion, and mesocosm experiments showed that bacteria were P limited in the epilimnion but not in the hypolimnion. Total algal and zooplankton densities increased after ice-out, while Daphnia and Bosmina densities decreased. Increases in zooplankton grazing rates after ice-off were most dramatic in small-bodied zooplankton. Sediment core analysis showed that Asterionella relative abundance continues to increase, suggesting that the lake has become more brackish and oligotrophic. Our findings suggest that TSL has undergone a transition from dimictic to meromictic conditions, and that continued salt inputs have altered the structure and function of this ecosystem.

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.