Congruent energy density trends of fish and birds reflect ecosystem change

Abstract

Lake Huron, the third largest freshwater lake in the world, has become increasingly oligotrophic since the early 1990s and is also currently experiencing substantial changes in species abundances and diversity. We used longterm biomonitoring data to calculate whole-body energy densities for Lake Huron rainbow smelt (Osmerus mordax), lake trout (Salvelinus namaycush), and also for herring gull (Larus argentatus) eggs collected from 1989-2011. Significant temporal declines in energy densities were observed for all three species, and significant declines were found for lake trout size-at-age and herring gull egg sizes. For lake trout and herring gulls, these declines occurred during a period of reduced abundance of pelagic forage fish and increased inclusion of terrestrial food in diets of herring gulls. Declining energy density in rainbow smelt occurred concomitant with declining phytoplankton and zooplankton resources, the principal energy source for this species. Reduced production capacity in lower trophic levels appears to be affecting upper trophic levels of the Lake Huron food web, emphasizing the cascading effects of ecosystem change.

Contrasting PCB bioaccumulation patterns among Lake Huron lake trout reflect basin‐specific ecology

“…The present study has demonstrated substantially differing patterns of PCB congener bioaccumulation and biomagnification among Lake Huron lake trout that predominantly feed on rainbow smelt and occupy the same trophic level but inhabit different basins within this ecosystem. Recent evidence suggests that broad scale changes have occurred in Lake Huron food web structure, with significant changes in species abundance, growth, and production observed across multiple trophic levels in this ecosystem [43][44][45]. Such a regime shift suggests that the results observed in the present study may not be limited solely to lake trout and could be manifested throughout the trophic levels of the Lake Huron food web.…”

Mercury biomagnification and contemporary food web dynamics in lakes Superior and Huron

“…Species codes are listed in Tables 2 and 3. For each species, a composite (n = 1) from N samples (Tables 2 and 3) the Lake Huron lake trout (Paterson et al, 2014). These changes have likely negatively impacted the growth efficiencies of the Lake Huron lake trout, yielding higher measured Hg levels over time.…”

Basin‐Specific Pollutant Bioaccumulation Patterns Define Lake Huron Forage Fish

“…For example, should prey communities, abundances, and food-web structures be similar across multiple compartments of the same ecosystem, this would be expected to manifest in similar magnitudes and patterns of congener bioaccumulation among individuals of the same species. That basin-specific PCB profiles have now been demonstrated for top predator (Paterson et al 2016) and secondary consumer forage fish species collected from across Lake Huron confirms conclusions regarding the importance of lower trophic level processes in regulating pollutant and energy transfer within the Lake Huron food webs (Bunnell et al 2014;Paterson et al 2014;McLeod et al 2019). Bunnell et al (2014) concluded that bottom-up control mechanisms predominate in terms of nutrient and energy availability and flow in Lake Huron.…”

“…For example, it was concluded that Lake Huron experienced a regime shift that included a collapse of the Main Basin alewife (Alosa pseudoharengus) population and a transition in top predator dominance from Chinook salmon (Oncorhyncus tschawytcha) to native lake trout (Salvelinus namaycush; Madenjian et al 2013;He et al 2015He et al , 2016. Further restructuring included a decline to ultra-oligotrophic nutrient status (Barbiero et al 2011;Cha et al 2011), reductions in early season primary productivity and altered zooplankton community structure to increasingly oligotrophic species (Barbiero et al 2012), declining forage fish abundances (Riley et al 2008;O'Brien et al 2018), and reduced consumer energy densities (Paterson et al 2014). However, the extent to which such ecological restructuring has occurred in the food webs from each of Lake Huron's 3 basins remains unknown.…”