29 Lakes, Climate, and Environmental Change: Paleolimnological Studies of the Holocene and “Anthropocene”

Friday, 28 March 2008
1:00 PM - 5:20 PM

LACUSTRINE SURFACE AND DEEP WATER NUTRIENT RESPONSES TO HISTORICAL DEFORESTATION

Paper number: 29-1
Program: THEME SESSIONS
Time: 1:00 PM – 1:20 PM

Authors

Abstract

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 19th 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.