Tracking the History and Ecological Changes of Rising Double-Crested Cormorant Populations Using Pond Sediments from Islands in Eastern Lake Ontario

<div><p>In the Laurentian Great Lakes region, the double-crested cormorant (<i>Phalacrocorax auritus</i>) has seen a thousand-fold population increase in recent decades. These large colonies of birds now often conflict with socioeconomic interests, particularly due to perceived competition with fisheries and the destruction of terrestrial vegetation in nesting habitats. Here we use dated sediment cores from ponds on islands in eastern Lake Ontario that receive waste inputs from dense colonies of cormorants and ring-billed gulls (<i>Larus delawarensis</i>) to chronicle the population rise of these species and assess their long-term ecological impacts. Modern water chemistry sampling from these sites reveals drastically elevated nutrient and major ion concentrations compared to reference ponds not influenced by waterbirds. Geochemical tracers in dated sediment cores, particularly δ¹⁵N and chlorophyll-<i>a</i> concentrations, track waterbird influences over time. Fossil diatom assemblages were dominated by species tolerant of hyper-eutrophic and polluted systems, which is in marked contrast to assemblages in reference sites. In addition to establishing long-term ecological impacts, this multi-proxy paleoecological approach can be used to determine whether islands of concern have been long-term nesting sites or were only recently colonized by cormorant or ring-billed gull populations across the Great Lakes, facilitating informed management decisions about controversial culling programs.</p></div

Cormorant impact and morphological characteristics of study ponds.

<p>Pond abbreviations are as follows: East Brother (EB), Pigeon (PGN), False Duck Pond 1 (FD1), and Main Duck Pond 2 (MD2). The arrival of cormorants to East Brother and False Duck is based on data from D.V.C. Weseloh (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0134167#pone.0134167.s002" target="_blank">S1 Table</a>) and based on data from [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0134167#pone.0134167.ref017" target="_blank">17</a>] for PGN. Numbers for nests, as well as pond diameter and depth, are estimates made in the field at the time of sampling. Details of water chemistry sampling methods and dates are provided in the materials and methods section.</p><p>Cormorant impact and morphological characteristics of study ponds.</p

Water chemistry for impacted and reference sites.

<p>(A) Specific conductivity (μS/cm) in high-impact ponds in black and hashed (ponds EB and PGN, respectively), the low-impact pond (FD1) in grey, and the no-impact pond (MD2) in white. Major cations and anions are listed in the inset by decreasing concentration. (B) Major nutrients in three study ponds, including total unfiltered nitrogen (TN) concentrations (mg/L) and total unfiltered phosphorus (TP) concentrations (μg/L).</p

Subfossil diatom assemblages and geochemical tracers from sediment cores.

<p>Data are shown for the high-impact ponds on (A) East Brother Island (EB) and (B) Pigeon Island (PGN), as well as from the reference sites on (C) False Duck Island (FD1) and (D) Main Duck Island (MD2). The cyst to diatom ratio (C:D), sedimentary δ¹⁵N (‰) and spectrally-inferred sedimentary chlorophyll-<i>a</i> (chl-<i>a</i>) concentrations (mg/g) are given to the right of each stratigraphy. All profiles are presented on the same scales, except the C:D ratio for PGN. Grey boxes at the bottom of each core indicate sediment intervals in which diatom remains were too sparse to be enumerated.</p

A map of eastern Lake Ontario near Kingston, ON.

<p>Study ponds (with name and impact level) are on East Brother Island (EB, high-impact), Pigeon Island (PGN, high-impact), False Duck Island (FD1, low-impact), and Main Duck Island (MD2, no-impact).</p