Our research approach combines biogeochemical methods with paleoecology to provide a long-term perspective on understanding natural environmental variability and recent anthropogenically-induced changes. Our research is located across a wide range of environments from the sub-Arctic and Arctic to Atlantic Canada to tropical coastal Thailand. These environments face many anthropogenic stressors from land-use changes to global warming, which can manifest in several ways within aquatic and terrestrial ecosystems. For example, northern ecosystems are undergoing significant changes with warming including increases in the length of the ice-free period, increased water column stability, the northward shifts of species ranges, increases in the rates of weathering and decomposition, changes in snowmelt, and shifting water balances, to name a few. In Southeast Asia, the rapid expansion of shrimp aquaculture in the last few decades has deteriorated coastal ecosystems including mangrove forests. Often many of these recent changes require knowledge on environmental conditions prior to the onset of impact or introduction of an ecosystem stressor, but monitoring records can be scarce, especially in inaccessible regions of the world. This is where the importance of paleolimnological and paleoecological records come in.
Seabirds as biovectors
A biovector is any organism that transports a substance (e.g., nutrient, contaminant, virus) from one ecosystem to another. In particular, birds feed almost exclusively off shore but nest on land, often forming large breeding colonies or roosts where they can number in the tens or even hundreds of thousands. The wastes released from bird colonies provide critical nutrient subsidies for many coastal ecosystems, often creating thriving biological communities that would not exist otherwise. An unfortunate irony is that this transport pathway also concentrates contaminants that are biomagnified and bioaccumulated through the marine or aquatic food webs, thereby threatening the very ecosystems it supports and sustains. For this research, we are working to develop many proxies, including metals, subfossil diatoms, and lipophilic biomarkers to understand the land-sea linkages created by biovectors. Examples of this research: Hargan et al. 2017, Stewart et al. 2019, Duda et al. 2018
A sediment core and eider pond, Hudson Strait
Our ongoing work falls into several themes:
Anthropogenic impacts to aquatic ecosystems including sediment contaminant analysis
Knowing historical baseline conditions in aquatic ecosystems is critical for setting realistic recovery targets and conservation goals. Many aquatic ecosystems may be naturally productive or cycle through different steady states (e.g., clear with macrophytes or turbid with algal blooms). Using aquatic ecological and paleolimnological studies our research strives to understand how algal and invertebrate assemblages respond to anthropogenic impacts including climate change, nutrient pollution, historical mining, and contaminants. Our research lab at Memorial currently has a GC-MS and is finessing methods for the analysis of different lipids and contaminants in sediments.
Examples of this research: Hargan et al. 2016, Thienpont et al. 2016, Stewart et al. 2018, Hargan et al. 2020
Piston coring on North Raft Lake, Hudson Bay Lowlands
Paleolimnology meets conservation biology
Long-term ecological and environmental studies are required to disentangle the natural range of an animal's population variability from relatively recent anthropogenically driven declines. Of the numerous declining animal populations, seabirds are particularly threatened and require long-term monitoring because of their ability to cross geographical and political boundaries, and thus experience varying effective and inconsistent management strategies. Here we use paleo-archives from seabird colonies to determine timing of colonization and reconstruct population fluctuations alongside independent archives of climatic change and historical records of anthropogenic impacts. These paleo-data can identify past avian distribution, colony longevity, and population trends, and have clear implications for predicting the potential impacts of future climatic and environmental change on avian species.
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A nesting Common eider; image credit: Grant Gilchrist
Examples of this work reconstructing seabird population trends: Hargan et al. 2019, Duda et al. 2020, 2021
Historical moth defoliator outbreak signals in lake sediments
Phytophagous insects, particularly taxa that experience outbreaks over large spatial extents, such as spruce budworm, cause significant forest disturbance. Spruce budworm have a large influence on primary and secondary productivity, with implications for ecosystem services provided by forests. Given the relatively long periods between spruce budworm outbreaks, we have few historical data to reconstruct outbreak dynamics prior to significant human settlement. However, spruce budworm wing scales preserve in lake sediment and therefore sediment cores can provide a window into budworm abundance 1000s of years from present. We are working to expand ongoing paleolimnological work in the province of NL using proxies in lake sediment cores as a measure of provincial forestry dynamics and spruce budworm abundance in deep time in order to understand the cyclical nature of outbreaks and correspondence to environmental cues (e.g., climate).
Poster by MSc student Courtney White
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A bog in Hudson Bay Lowlands
Examples of this research: Hargan et al. 2015 Botany, Hargan et al. 2015 The Holocene
Environmental gradients across wetlands and drivers of past ecological change
With climate change, studies of northern peatlands are now more important than ever owing to the vast quantities of carbon stored within these regions and their future role as net carbon sinks. All biological indicators are limited by their responses to environmental factors (e.g. moisture, pH, etc.), and these responses will differ among proxies. A multi-proxy approach allows comparisons and corroboration among the indicators, thereby strengthening paleoecological interpretations. We are studying the response of diatoms across northern peatlands to small differences in microhabitats, including varying pH, moisture, and bryophyte type. In conjunction with other proxies, this knowledge on modern diatom autecology will improve our interpretations on past environmental changes captured in peat archives.
A mangrove forest in eastern Thailand
The influence of aquaculture effluent within wetlands and marine ecosystems
Mangrove forests perform many functions that are beneficial to people, including buffering excess nutrients received from land-use activities, and storing carbon as organic matter. However, few studies have evaluated the linkages between excess nutrient runoff from aquaculture and the capacity of mangrove forests to utilize these nutrients to store greater carbon. For this research we are utilizing a comprehensive suite of chemical tracers in a coupled aquaculture-marine ecosystems to evaluate the impact of aquaculture runoff on coastal nutrient dynamics. We aim to: 1) characterize across multiple levels in the food web the relative contributions of carbon and nitrogen from aquaculture and marine-derived nutrients; and 2) use sediment cores to reconstruct the relative change in nutrient and carbon sources and storage over time.
Examples of this research: Hargan et al. 2020 in Journal of Applied Ecology, Hong et al. 2020 in Ecological Indicators