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

Sediment cores from ponds on waterbird nesting islands in Lake Ontario were assessed for 1) subfossil diatom assemblages, 2) stable nitrogen isotopes, and 3) sediment chlorophyll a in order to reconstruct possible historical use of these islands by nesting Double-crested Cormorants and/or Ring-billed Gulls. Increases in the relative abundance of eutrophic diatom species, in conjunction with elevated ratios of stable nitrogen isotopes and increased sediment chlorophyll a, reflect large inputs of guano and thus effectively track waterbird influence. These data were compared with previously analysed sedimentary sterols and stanols (Hargan et al. 2018 Proc. R. Soc. Lond. B. 285: 20180631) in order to separate the influences of cormorants from gulls. Collectively, the sedimentary proxies indicate that cormorants were unlikely to have used these islands in such large numbers (as are found today) in the past ~200 or more years. Ring-billed Gulls, however, have likely used current nesting islands for much of the 20th century. Data of the radioisotopic dating of the sediment cores can be found in Hargan et al. (2018 Proc. R. Soc. Lond. B. 285: 20180631) supplemental material.