LACUSTRINE SURFACE AND DEEP WATER NUTRIENT RESPONSES TO HISTORICAL DEFORESTATION

Fayetteville Green Lake is a small (0.3 km², 52 m deep), salinity-stratified meromictic lake located near Syracuse, NY. The stratification creates two water masses with distinct populations of phototrophs: those in the nutrient-limited, oxygenated, surface waters and those in the nutrient-replete, sulfidic, but light-limited waters below 20 m depth. These two populations assimilate nitrogen from different pools, with nitrate available in surface waters and ammonium abundant in deep waters. In this study we assess changes in deep water nutrient conditions following deforestation of the watershed in the early 19^th century. We use novel compound-specific stable isotope techniques to show that the relatively transient increase in surface water productivity stimulated by soil nutrient release had long-term effects on deep water chemistry. This method may also be useful in studying productivity during the deposition of black shales under anoxic conditions.

In the modern Green Lake water column, prokaryotes living below the chemocline have δ¹⁵N values between -0.3 and -3.6 ‰. These values are ¹⁵N-depleted relative to ammonium remineralized in surface sediments of the deep basin (δ¹⁵N = -0.1 ‰). The upward diffusing ammonium that is not assimilated below the chemocline is ¹⁵N-enriched (+13.4 to +23.7 ‰) and helps maintain the ¹⁵N-enriched values of surface water nitrate (~8 ‰) through nitrification. Bacterial pigments produced by Green and Purple Sulfur Bacteria (GSB and PSB) below the chemocline have ¹⁵N-depleted δ¹⁵N values. The bacterial pigments also have δ¹³C values consistent with the carbon assimilation pathways of PSB and GSB. Fossil pigments extracted from a deep basin sediment core reveal positive excursions in δ¹⁵N and δ¹³C values following a short peak in surface water productivity ~200 years ago. The excursions persist for longer than 100 years after the initial pulse of soil nutrients, showing that deep water ammonium and dissolved inorganic carbon pools both responded to an increase in the flux of sinking biomass and its subsequent remineralization below the chemocline.