The influence of different top predators on nutrient and energy flow in aquatic systems
Quantifying in situ nutrient and energy flows in aquatic ecosystems represents an important ecological challenge, especially as anthropogenic change is altering aquatic systems. Nowhere is this more important than Lake Huron which is experiencing substantial change including ultra-oligotrophication of the lake, regime shifts in the pelagic fish community, invasive species, and fishing pressures. The influence of these stressors have affected the lake’s top predators causing decreased growth rates of Lake Trout populations, increasing the dominance of older (age 5+) individuals, and causing substantial declines in the Pacific Salmon abundances. Previously, we proposed the use of polychlorinated biphenyls (PCBs) as a metric to track the number of meals eaten by a fish and hence monitor nutrient and energy flow in aquatic systems. We further this concept using a non-steady state PCB bioaccumulation model to quantify nutrient and energy flows in both Pacific Salmon and Lake Trout to contrast the influence of life history traits and growth rates on nutrient and energy dynamics in Lake Huron.
The results of this study demonstrate that Lake Trout are more effective at recycling nutrients and are critical for food web stability in these highly oligotrophic ecosystems, whereas Pacific salmon tend to act mostly as nutrient sinks. As fish reach their asymptotic length of the von Bertalanffy growth curve, the mass of nutrients they recycle increases. For Lake Trout, at approximately 5 years of age, their individual growth rates fall below 50% yr-1, causing these upper age cohorts to become nutrient sources rather than sinks. Pacific Salmon, on the other hand, migrate to tributaries to spawn when individual growth rates decline below 50% yr-1 exporting significant masses of nutrients out of the lake. Given the differing life-spans, growth rates and reproductive strategies of Lake Trout relative to stocked Pacific salmonids in the Great Lakes, the results of this study demonstrate that Lake Trout provide a critical ecosystem service by effectively recycling nutrients to enhance food web stability in highly oligotrophic ecosystems.