The impacts of waterbird-mediated elemental enrichment on chironomid assemblages from island ponds in Lake Ontario

Ring-billed gulls (Larus delawarensis) and double-crested cormorants (Phalacrocorax auritus) have nested in dense colonies across the Laurentian Great Lakes since their rapid population expansions during the early- and mid-20 th century, respectively. Dated sediment cores from ponds located near nesting sites have been used to track the history of these large colonies on islands in eastern Lake Ontario. However, less is known about avian-mediated biotransport of contaminants to these nesting sites and its effects on aquatic biota at mid-trophic levels, such as chironomids. Here, we used a paleolimnological approach to determine the extent of possible avian- mediated metal(loid) contamination of ponds in nesting sites compared to reference sites. Water from avian-im- pacted ponds had higher concentrations of Al, Cd, Cu, Fe, Pb, and Se than the reference ponds, and exceeded the Canadian Council of Ministers of the Environment (CCME) guidelines for the protection of aquatic life at some sites. Down-core geochemical analysis of elements in sediments showed that concentrations of Cd, Se, and Zn were significantly higher in impacted ponds than in reference sites and correlated significantly with stable nitrogen isotopes, a marker of avian-derived nitrogen. In sites impacted by ring-billed gulls, Cd and Zn concentrations in sediments exceeded or approached those at which frequent adverse biotic effects are expected. Metal contamination was linked to altered chironomid assemblages, with subfossil remains in the avian-impacted sites including more taxa tolerant of heavy metal contamination than reference sites. Notably, chironomids were largely absent from the sites with the highest metal concentrations. These findings have implications for understanding and managing contaminant cycling in the Great Lakes, as waterbirds that nest in dense colonies can act as biovectors that transport metals to new and unexpected locations.

Is there a common threshold to subfossil chironomid assemblages at 16 m water depth? Evidence from the Tibetan Plateau

Fluctuating lake levels are an important driver of ecosystem change, and changes in the precipitation/evaporation balance of a region can lead to undesirable changes in ecosystem functioning. Large-scale changes in hydrology will become increasingly more likely as a result of ongoing climate change in the coming century. This is especially true for the Tibetan Plateau, which plays a crucial role as the “Asian water tower” for the surrounding densely populated regions. Chironomids (Diptera: Chironomidae) have proven to be one of the most valuable bioindicators for monitoring and reconstructing the development of aquatic ecosystems. Besides temperature, water depth and salinity are two of the most important environmental factors affecting chironomids. To study the relationship between chironomids and water depth, we analyzed surface sediment samples of two large Tibetan lakes, Selin Co and Taro Co. These lakes have similar environmental conditions (e.g. elevation, temperature and oxygenation) but show strong differences in salinity (7–10 and 0.5 ppt, respectively). Our results show that the chironomid assemblages in both lakes have similar water depths at which the fauna abruptly changes in composition, despite different faunal assemblages. The most important boundaries were identified at 0.8 and 16 m water depth. While the uppermost meter, the “splash zone”, is characterized by distinctly different conditions, resulting from waves and changing water levels, the cause of the lower zone boundary remains enigmatic. Even though none of the measured water depth-related factors, such as water temperature, oxygen content, sediment properties, light intensity or macrophyte vegetation, show a distinct change at 16 m water depth, comparison to other records show that a similar change in the chironomid fauna occurs at 16 m water depth in large, deep lakes around the world. We propose that this boundary might be connected to water pressure influencing the living conditions of the larvae or the absolute distance to the surface that has to be covered for the chironomid larvae to hatch. We conclude that water depth either directly or indirectly exerts a strong control on the chironomid assemblages even under different salinities, resulting in distribution patterns that can be used to reconstruct past fluctuations in water depths.

Some aspects of the ecological niche of chironomids associated with submersed aquatic macrophytes in a tailwater

High values of chironomids' production were recorded and analysed in the tailwater of a dam reservoir located on a large river. The values resulted from submersed aquatic macrophytes (SAM) patches that appear on the bottom in summer, as an effect of a specific discharge pattern through the dam. Abundant and taxonomically rich chironomid assemblages develop there, but their populations display different spatial and trophic preferences, which are the main topic of our interest here. We focused on dominant taxa of Orthocladius/Cricotopus spp., Chironomus riparius Meigen and Glyptotendipes cauliginellus (Kieffer), whose abundance and biomass exceeded 90% of all chironomids inhabiting the SAMs. The developmental time of their aquatic stages (cohort production interval − CPI) and thus production, and production to biomass ratio (P:B) differed considerably. A P:B ratio of 13.8 was estimated for the first taxon, a mid-body size scraper living on stems and leaves of SAMs. In contrast, P:B ratios of 5.2-7.7 were assessed for the second and third taxa, rather large body size benthic gathering collectors inhabiting the roots of SAMs. The main food categories of gathering collectors were particulate organic matter trapped by the roots, while scrapers fed on algae developing on leaves and stems.