Freshwater food web biomanipulation: A powerful tool for water quality improvement, but maintenance is required

Experiences with lake food web biomanipulation have involved three basic biomanipulation types, repeated in two freshwater environments with distinctly different physio-chemical characteristics. The manipulation types are planktivore additions, complete planktivore removals and planktivore control through piscivore addition. The efficacy of these manipulations is strongly dependent on whether the aquatic environments comprise deep lakes that stratify, or shallow lakes and ponds that are not subject to extended periods of summer stratification. The general results of these six biomanipulation categories are that planktivore additions to stratified lakes almost invariably result in reduced grazer biomasses and increased algal densities. Planktivore additions in shallow lakes or ponds also result in more algae, with the modes of action including: increased bioturbation, decreased grazer biomass and reductions in macrophytes. Planktivore removals in deep stratified lakes frequently result in initial food web-mediated reductions in algal standing stocks, but given sufficient time, ungrazable algae often dominate and total algal biomasses return to premanipulation conditions. Planktivore removals in shallow lakes or ponds tend to be quite stable due to initial decreases in bioturbation and increases in macrophyte cover and zooplankton grazing pressure. However, long-term studies suggest that even these initially very successful manipulations may be subject to deterioration due to macrophyte species substitutions, reinvasion by planktivore-benthivores, and algal species substitutions. The emerging consensus is that in most cases ‘some maintenance is required’. Piscivore additions to deep lakes that stratify, seldom result in more than short periods of algal control. Species substitutions among invertebrate predators, the proliferation of ungrazable algae and changes in nutrient recycling, frequently produce new restructured food webs capable of sustaining algal biomasses that rival those observed during premanipulation periods. Piscivore additions in shallow lakes and ponds have been more successful, but only as long as planktivore-benthivore biomass remains low. Success is further enhanced when macrophyte communities remain intact. However, long-term studies suggest that piscivore densities often decline and that piscivores seem unable to maintain the planktivore-benthivore community at the very low levels that are required. Again the emerging consensus is that in most cases, maintenance is required. None of this invalidates the biomanipulation principle. Many biomanipulations have produced quite spectacular initial results and a few have maintained low algal biomasses for several years. Monitoring and maintenance seems a small price to pay. Given that all of the alternatives to biomanipulation (i.e. sewage treatment in all its forms) are much more management-intensive and expensive, and given that biomanipulations can be applied to non-point source situations that cannot be treated with traditional engineering-based methods, the prospect of having to maintain biomanipulations remains, on balance, quite acceptable. In conjunction with nutrient abatement and with the implementation of well planned maintenance programmes, the cost benefits of biomanipulation seem unassailable.     

An exploratory investigation of the landscape-lake interface: Land cover controls over consumer N and C isotopic composition in Lake Michigan rivermouths

Rivermouth ecosystems are areas where tributary waters mix with lentic near-shore waters and provide habitat for many Laurentian Great Lakes fish and wildlife species. Rivermouths are the interface between terrestrial activities that influence rivers and the ecologically important nearshore. Stable isotopes of nitrogen (N) and carbon (C) in consumers were measured from a range of rivermouths systems to address two questions: 1) What is the effect of rivermouth ecosystems and land cover on the isotopic composition of N available to rivermouth consumers? 2) Are rivermouth consumers composed of lake-like or river-like C? For question 1, data suggest that strong relationships between watershed agriculture and consumer N are weakened or eliminated at the rivermouth, in favor of stronger relationships between consumer N and depositional areas that may favor denitrification. For question 2, despite apparently large riverine inputs, consumers only occasionally appear river-like. More often consumers seem to incorporate large amounts of C from either the nearshore or primary production within the rivermouth itself. Rivermouths appear to be active C and N processing environments, thus necessitating their explicit incorporation into models estimating nearshore loading and possibly contributing to their importance to Great Lakes biota.     

Ecological tracers track changes in bird diets and possible routes of exposure to Type E Botulism

Dreissenid mussels have become important components of the Great Lakes biological community since their introduction in the 1980s, but much remains to be understood regarding their effect on energy and nutrient flows in pelagic systems. Here, we report a new method that tracks incorporation of resources of molluskan origin into food webs used by aquatic birds. Biochemical tracers (fatty acids and stable carbon isotopes) are used to characterize species associated with pelagic and benthic food webs in Lake Ontario. Our focus is on the polymethylene-interrupted fatty acids (PMI-FAs) because previous research identified mollusks as their primary source. We found that PMI-FA mass fractions were greater in organisms associated with benthic (e.g. round goby) versus pelagic (e.g. alewife) food webs. Double-crested cormorants that had recently consumed benthic prey fish, i.e. goby, had greater proportions of PMI-FAs in their blood plasma than birds which showed no signs of recent goby ingestion. We did not detect an increase in mass fractions of PMI-FAs in cryogenically archived cormorant eggs following expansion of dreissenid mussels in Lake Ontario. However, following the introduction and expansion of round goby in the lake, PMI-FAs were detected at greater levels in cormorant eggs. These results illustrate how only after dreissenid mussel-facilitated establishment of round goby was the full extent of exotic species disruption of food webs manifested in fish-eating birds. These food web changes may be contributing to negative impacts on aquatic birds exemplified by the emergence of Botulism Type E as a significant mortality factor in this ecosystem.     

Effect of dam removal on habitat use by spawning Atlantic salmon

By impeding migration and degrading habitat downstream, dam construction has caused population declines in many migratory fish populations. As part of the landlocked Atlantic salmon (Salmo salar) restoration program in Lake Champlain, the Willsboro Dam was removed from the Boquet River, NY in 2015 providing an opportunity to study the effects of dam removal on spawning habitat quality and availability. Spawning habitat surveys were conducted downstream of the dam site in 2014, 2016 and 2017, and in historical spawning grounds upstream in 2016 and 2017. The habitat used was characterized by measuring depth, water velocity, and substrate size at each redd. Mean habitat use did not differ between upstream and downstream sites for any variables in 2016 and only differed for depth in 2017. However, the variance in depth and substrate used for spawning were lower at the upstream site in 2016, likely due to an abundance of habitat. In the downstream site, the mean and variance in depth at redds decreased after dam removal as did the variance in substrate size, increasing the habitat suitability of redds. When compared to literature data, habitat used upstream of the former dam was of medium quality in both 2016 and 2017, and improved downstream from low to medium quality in both column velocity and substrate size after dam removal. This study illustrates that positive shifts in the quality of habitat used can occur rapidly following dam removal by allowing access to suitable spawning habitat upstream and improving habitat downstream.