Atmospheric deposition is a primary source of reactive nitrogen (Nr) to undisturbed watersheds of the Great Lakes region of the U.S., raising concerns over whether enhanced delivery over recent decades has affected lake ecosystems. The National Atmospheric Deposition Program (NADP) has been measuring Nr deposition in this region for over 35 years. Here we explore the relationships among NADP-measured Nr deposition, nitrogen stable isotopes (δ 15 N) in lake sediments, and the response of algal communities in 28 lakes situated in national parks of the western Great Lakes region of the U.S. We find that 36% of the lakes preserve a sediment δ 15 N record that is statistically correlated with some form of Nr deposition (total dissolved inorganic N, nitrate, or ammonium). Furthermore, measured long-term (since 1982) nitrogen biogeochemistry and inferred critical nitrogen loads suggest that watershed nitrogen retention and climate strongly affect whether sediment δ 15 N is related to Nr deposition in lake sediment records. Measurements of algal change over the last~150 years suggest that Nr deposition, in-lake nutrient cycling, and watershed inputs are important factors affecting diatom community composition, in addition to direct climatic effects on lake physical limnology. The findings suggest that bulk sediment δ 15 N does reflect Nr deposition in some instances. In addition, this study highlights the interactive effects of Nr deposition and climate variability.

Abstract.In regions with airborne contaminants and large snowpacks, there is concern over the impact that snowmelt chemical "pulses" -periods of sharp increase in meltwater solute concentration -could have on aquatic resources during spring runoff. A major variable in determining such an effect is the flow path of snowmelt solutes to the stream or lake. From December 1988, to late April 1989, the quality and quantity of precipitation, snowmelt, soil solution and streamwater were measured in a 176-ha gauged watershed on the south shore of Lake Superior. The main objectives were to (1) examine the change in flow path meltwaters take to the stream during distinct winter and spring hydrologic periods, (2) quantify ecosystem-level ion budgets prior to, during, and following snowmelt, and (3) examine if streamwater chemistry might be a sensitive indicator of change in ecosystem flow paths. Prior to peak snowmelt, groundwater made up 80% of stream discharge. During peak snowmelt, the groundwater level rose to the soil surface resulting in lateral water movement through near-surface macropores and as overland flow. Near the end of snowmelt, mehwaters exerted a piston action on deeper soil solution again increasing its relative contribution to streamwater discharge. Net groundwater drawdown during the study resulted in streamwater discharge about equal to precipitation inputs. Unfrozen soils and brief midwinter thaws resulted in steady snowmelt throughout early and mid-winter. The snowpack lost > 50% of most ions prior to the period of major snowmelt and high stream discharge in late March and early April. Snowmelt and streamwater NO; and NH: pulses occurred before the period of overland flow and peak streamwater discharge (April 4-24). During overland flow, stream discharge of total N, P, DOC, and AI peaked. Nutrient budgets computed for distinct hydrologic periods were much more helpful in explaining ecosystem pathways and processes than were changes in solute concentration. For the study period, watershed base cation (C,) discharge was 23 times input and SO:-discharge exceeded input by 42%. H+ was the most strongly conserved ion with output <0.2% of input. Also conserved were NH: with only 1.4% of input leaving the ecosystem and NO; with output equal to 9.4% of input.

Since 1987 we have studied weekly change in winter (December-April) precipitation, snowpack, snowmelt, soil water, and stream water solute flux in a small (176-ha) Northern Michigan watershed vegetated by 65-85 year-old northern hardwoods. Our primary study objective was to quantify the effect of change in winter temperature and precipitation on watershed hydrology and solute flux. During the study winter runoff was correlated with precipitation, and forest soils beneath the snowpack remained unfrozen. Winter air temperature and soil temperature beneath the snowpack increased while precipitation and snowmelt declined. Atmospheric inputs declined for H + , NO 3 À , NH 4 + , dissolved inorganic nitrogen (DIN), and SO 4 2À . Replicated plot-level results, which could not be directly extrapolated to the watershed scale, showed 90% of atmospheric DIN input was retained in surface shallow (<15 cm deep) soils while SO 4 2À flux increased 70% and dissolved organic carbon (DOC) 30-fold. Most stream water base cation (C B ), HCO 3 À , and Cl À concentrations declined with increased stream water discharge, K + , NO 3 À , and SO 4 2À remained unchanged, and DOC and dissolved organic nitrogen (DON) increased. Winter stream water solute outputs declined or were unchanged with time except for NO 3 À and DOC which increased. DOC and DIN outputs were correlated with the percentage of winter runoff and stream discharge that occurred when subsurface flow at the plot-level was shallow (<25 cm beneath Oi). Study results suggest that the percentage of annual runoff occurring as shallow lateral subsurface flow may be a major factor regulating solute outputs and concentrations in snowmelt-dominated ecosystems.

The 115‐ha boreal Wallace Lake watershed, located on Isle Royale in the Lake Superior Basin, receives moderate anthropogenic atmospheric N inputs (3 kg ha−1 yr−1). Ecosystem response to atmospheric N inputs is, in part, determined by internal production and consumption of this limiting nutrient. The objectives of this 2‐yr study on plots dominated by birch, spruce, or alder were to quantify forest floor and surface soil net and gross N mineralization rates, and examine potential effects of changes in temperature and moisture on these processes. Gross N mineralization rates were 23 times net mineralization rates, and increased with forest floor/soil temperature and moisture. Substrate quality was a likely factor in higher gross mineralization rates beneath birch and spruce. Ammonium immobilization increased with forest floor/soil temperature and moisture. Higher net N mineralization rates beneath alder resulted from lower microbial immobilization rather than greater gross N mineralization. The greatest differences between gross N mineralization and immobilization occurred in early summer. Ammonium immobilization averaged 62% of gross N mineralization. Net and gross nitrification rates differed by vegetation type, were highest in spring and fall, and increased with moisture. Gross nitrification was 19 times net nitrification rates. Nitrate immobilization increased with soil moisture, and equalled or exceeded gross nitrification. Net and gross N mineralization or nitrification rates were not correlated. Seasonal variation in forest floor and surface soil N cycling coupled with high ecosystem retention of precipitation N inputs suggest streamwater N concentrations and output reflect trends in soil processes.

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