Pond sediments on nesting islands in eastern Lake Ontario provide insights into the population dynamics and impacts of waterbird colonies

Marked increases in ring-billed gulls and double-crested cormorants in the Laurentian Great Lakes during the last century have garnered attention over their ecological impacts, sparking debates about management strategies. However, monitoring data are generally sparse and of short duration for these colonies, hampering the ability to place recent changes within long-term context. Sediment records from ponds immediately surrounded by colonies on nesting islands can be used to track past bird populations, as they release wastes with geochemical signals that can be tracked using traditional paleolimnological methods, such as diatoms and stable isotopes of nitrogen (δ¹⁵N). Here, we provide new information about waterbird nesting histories on islands in eastern Lake Ontario that are of interest to wildlife managers. In all bird-impacted ponds, eutrophic diatom assemblages and elevated sediment chlorophyll-a coincided with high signatures of δ¹⁵N in the recent sediments, signifying bird influence. An absence of significant bird impacts in the oldest portion of one sediment core indicates that the current cormorant colony size (>2000 birds) is unprecedented over the ~150-year record. Diatoms and sediment chlorophyll-a also responded to even a small, short-duration cormorant nesting event on an actively managed reference island. Collating our findings with those from four previously studied sediment cores from nearby islands, we show that cormorants are unlikely to have occupied eastern Lake Ontario in their current numbers within the past ~150?years. However, ring-billed gulls have likely used several islands in the area for most of the 20th century (and perhaps earlier) until present.