Surface water connectivity of Arctic lakes drives patterns of fish species richness and composition, and food web structure

Abstract

Hydrological processes regulate fish habitat, largely controlling availability and suitability of habitat for freshwater fishes. Seasonal fluctuations in surface water distribution and abundance on the Arctic Coastal Plain, Alaska, influence individual fish species occupancy in lentic habitats. On low-relief tundra, permafrost processes and climate are chiefly responsible for lake formation and surface water dynamics, such as the timing, duration, and availability of water that affects fish species distributions. However, it is unclear how these relationships scale up to influence fish community richness and composition, or food web structure. Further, each of these processes is also likely to change with rapid climate warming occurring in the Arctic. By observing patterns of fish species occupancy, we examined how fish species richness and composition in Arctic lakes varied with surface water connectivity at coarse and spatial fine scales. Through experiments and observation, we determined the structure of food webs as they related to surface water connectivity and foraging habits of associated fish species. We found surface water connectivity was a driver of fish species richness and assemblage patterns. Permanently connected lakes contained nearly twice as many species as disconnected lakes; and the most strongly connected lakes contained an average of four additional species compared to isolated lakes. Functional traits of fishes, like life history or body morphology, likely dictate their ability to colonize habitats. Given reduced colonization potential, isolated lakes either never supported or could not retain larger predatory fishes. In isolated systems only one fish predator occurred consistently, and this species showed strong top-down control of invertebrate prey in experimental systems. Yet, in natural environments single-predator systems have fewer trophic links than multi-predator systems, and therefore, less trophic redundancy across species. The loss of species due to isolation reduced the total number of trophic links and shortened food chains. However, I argue that the complexity and addition of top-predators in surface water connected lakes adds trophic redundancy, stabilizes energy flow, and potentially enhances persistence within in food webs and across the meta-community of food webs. Changes to fish species richness, composition, or food web structure from climate warming may be dampened by the resilience of food webs locally, but across the broader landscape it is likely that some food webs will be restructured due to changes in colonization potential regulated by surface water connectivity.