Adam J. Heathcote

Large increases in carbon burial in northern lakes during the Anthropocene

Northern forests are important ecosystems for carbon (C) cycling and lakes within them process and bury large amounts of organic-C. Current burial estimates are poorly constrained and may discount other shifts in organic-C burial driven by global change. Here we analyse a suite of northern lakes to determine trends in organic-C burial throughout the Anthropocene. We found burial rates increased significantly over the last century and are up to five times greater than previous estimates. Despite a correlation with temperature, warming alone did not explain the increase in burial, suggesting the importance of other drivers including atmospherically deposited reactive nitrogen. Upscaling mean lake burial rates for each time period to global northern forests yields up to 4.5 Pg C accumulated in the last 100 years—20% of the total burial over the Holocene. Our results indicate that lakes will become increasingly important for C burial under future global change scenarios.

Watershed Sediment Losses to Lakes Accelerating Despite Agricultural Soil Conservation Efforts

Agricultural soil loss and deposition in aquatic ecosystems is a problem that impairs water quality worldwide and is costly to agriculture and food supplies. In the US, for example, billions of dollars have subsidized soil and water conservation practices in agricultural landscapes over the past decades. We used paleolimnological methods to reconstruct trends in sedimentation related to human-induced landscape change in 32 lakes in the intensively agricultural region of the Midwestern United States. Despite erosion control efforts, we found accelerating increases in sediment deposition from erosion; median erosion loss since 1800 has been 15.4 tons ha−1. Sediment deposition from erosion increased >6-fold, from 149 g m−2 yr−1 in 1850 to 986 g m−2 yr−1 by 2010. Average time to accumulate one mm of sediment decreased from 631 days before European settlement (ca. 1850) to 59 days mm−1 at present. Most of this sediment was deposited in the last 50 years and is related to agricultural intensification rather than land clearance or predominance of agricultural lands. In the face of these intensive agricultural practices, traditional soil conservation programs have not decelerated downstream losses. Despite large erosion control subsidies, erosion and declining water quality continue, thus new approaches are needed to mitigate erosion and water degradation.

Reconstructing epilimnetic total phosphorus using diatoms: statistical and ecological constraints

Diatoms respond rapidly to eutrophication and diatom-based models for inferring total phosphorus (TP) have found wide application in palaeolimnology, especially in tracking trajectories of past and recent nutrient enrichment and in establishing pre-disturbance targets for restoration. Using new analysis of existing training sets and sediment-cores we examine the statistical and ecological constraints of diatom-inferred TP (DI-TP) models. Although the models show an apparently strong relationship between measured and inferred TP in the training sets, even under cross-validation, the models display three fundamental weaknesses, namely (1) the relationship between TP and diatom relative abundance is heavily confounded with secondary variables such as alkalinity and lake depth, (2) the models contain many taxa that are not significantly related to TP, and (3) comparison between different models shows poor or no spatial replicability. At some sites the sediment-core diatom assemblage change tracks the TP gradient in the training sets and DI-TP reconstructions are consistent with monitored TP data and known catchment histories for the recent past. At others diatom species turnover is apparently related to variables other than TP, and DI-TP fails to even reproduce plausible trends. Pre-disturbance DI-TP values are also questionable at most sites. We argue that these problems pervade many DI-TP models, particularly those where violations of the basic assumptions of the transfer function approach are ignored.

Phytoplankton taxonomic compositional shifts across nutrient and light gradients in temperate lakes

Abstract Nutrient and light availability, and their balance, can modify community composition and structure in pelagic communities. Previous studies have demonstrated contradictory findings about whether total phosphorus (TP) concentrations alone or the ratio of total nitrogen (TN) to TP concentrations (TN:TP) drive Cyanobacteria dominance in freshwater ecosystems, and influences of light availability are often overlooked. Here, we analyzed a 12 year, 137 lake database to test paradigms of phytoplankton compositional patterns across nutrient (TN, TP, TN:TP) and light availability gradients in an agricultural region. We hypothesized that (1) TN:TP ratios would better predict phytoplankton compositional shifts than TP concentrations alone, (2) Cyanobacteria relative abundance would increase at low TN:TP ratios, and (3) Cyanobacteria biomass fluctuations would be the primary driver of light climate. We found that TN:TP ratios better described phytoplankton compositional patterns than TP concentrations, with Cyanobacteria proportions decreasing with increasing TN:TP while other taxa increased. Contrary to expectations, Cyanobacteria always dominated community composition (≥80% biomass), regardless of TP concentrations. Despite these patterns, N-fixing Cyanobacteria proportions were not correlated to TN:TP, suggesting that shifts toward N-fixation were not solely driving phytoplankton compositional patterns. Although Cyanobacteria biomass decreased with increasing light availability, inorganic particles explained more variance in light than total phytoplankton biomass, suggesting that buoyancy-regulating Cyanobacteria may gain an initial competitive advantage in light acquisition before bloom development in turbid systems. These findings suggest that Cyanobacteria strongly influence pelagic community dynamics in nutrient-enriched lakes, and their ability to manipulate light and nutrient environments enable their persistent dominance across large environmental gradients.