Dreissenid Mussels and Large Lakes: Effects on Littoral Ecology
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Invasive organisms are one of the major threats to the ecological integrity of aquatic systems in the 21st century. Among the most notorious and important aquatic invasive organisms are the dreissenid mussels, Dreissena polymorpha and D. rostriformis bugensis, which having originated in the Ponto-Caspian region are now common in many parts of Europe and North America. Dreissenids have large impacts on many aspects of lentic ecosystem functioning, the sum of which is thought to lead to the translocation of biological production from the pelagic to the littoral zones of lakes. In this thesis I explore the effects of dreissenids on the nearshore zones of large lakes, investigate the mechanisms by which dreissenids couple the pelagic and nearshore zones of lakes and attempt to elucidate the factors affecting the strength of the dreissenid-mediated connection between the pelagic and littoral zones. The effects of invasive organisms on an aquatic ecosystem will depend, in part, on the distribution and biomass of the invasive organisms in the system. In chapter 2 I present the results of a lake-wide survey of the distribution of invasive dreissenid mussels in Lake Simcoe, Ontario and discuss some of the factors that shape their distribution pattern in the lake. Dreissenid biomass averaged 27.2 (±24.3 SD) g shell-free dry mass (SFDM)/m2 in the main basin of Lake Simcoe and 12.4 (±16.9 SD) g SFDM/m2 in macrophyte-dominated Cook’s Bay. I argue that water movement is an important determinant of dreissenid distribution, both through catastrophic disturbance in shallow water and through non-catastrophic effects on substrate distribution and possibly food supply rates. In areas of dense macrophyte growth, mussel abundance was shown to be associated with that of preferred macrophyte taxa, in particular with that of Ceratophyllum demersum. I used the results of my survey and the relationships between environmental variables and dreissenid biomass to estimate the total biomass of dreissenids in Lake Simcoe: 12,000 tonnes SFDM. Most of the dreissenid biomass in Lake Simcoe was concentrated in the nearshore zone, where dreissenids would have maximal impacts on littoral biological production. One of the effects of the dreissenid invasion into the Laurentian Great Lakes appears to be a resurgence in the abundance of the nuisance alga Cladophora glomerata which experienced a marked decline following phosphorus abatement in the late 1970s and early 1980s. A subsidy of bioavailable phosphorus excreted by dreissenid mussels could be an important mechanism facilitating the growth of C. glomerata. In chapter 3, I describe a survey of dreissenid distribution and abundance followed by in situ experiments designed to measure dreissenid phosphorus excretion rates. Average dreissenid mussel abundance in our study area was 3674 (±2233 SD) individuals/m2, with an average biomass of 52.2 (±29.0 SD) g of shell free dry mass/m2. The mussels excreted bioavailable soluble reactive phosphorus (SRP) at an average rate of 7.0 μg SRP/g shell free dry mass/hour, contributing about 11 tonnes of SRP to the study area over the C. glomerata growing season. Dreissenids appear to be an important source of recycled bioavailable phosphorus to the littoral zone, potentially supplying more soluble reactive phosphorus to the study area than local watercourses and waste water treatment plants, and more phosphorus than is required to sustain local C. glomerata growth. Dreissenid establishment in many systems coincides with increases in the abundance and diversity of littoral benthic invertebrates and with changes to community composition of the benthos. Currently, there is a lack of long-term studies of the impact of dreissenid mussels on hard-substrate inhabiting littoral benthos. In chapter 4 I compare the littoral benthos of Lake Simcoe, Ontario just prior, and 14 years following the establishment of dreissenids in the lake. Densities of non-dreissenid invertebrates on hard substrata increased by nearly 50 times, from an average of 367.9 (±460.8 SD) individuals/m2 in 1993 to an average of 16,706.4 (±10,204.5 SD) individuals/m2 in 2008. The taxonomic diversity of the benthos increased significantly. The distribution of benthic organisms also changed; the numerical abundance of benthos has become more even across depths and sites, as has community composition. I suggest that in addition to increasing resource availability to benthic organisms dreissenids have also caused a homogenization of the littoral habitat by increasing the evenness of the distribution of food and habitat resources. The changes in the littoral benthic community in Lake Simcoe likely have wide-ranging implications to higher trophic levels and the cycling of energy in the lake. In addition to impacting nutrient cycling and the benthic invertebrate communities of littoral zones, dreissenid mussels can have large effects on food webs and energy cycling. In chapter 5 I used stable isotope analysis of pre- and post-dreissenid components of the nearshore food web of Lake Simcoe, Ontario to determine how dreissenids affected food sources and energy flow in the littoral zone of Lake Simcoe. Results suggest that the post-dreissenid food web relies about equally on two energy sources: dreissenid biodeposits (redirected pelagic primary production) and littoral benthic primary producers. Although the relative importance of pelagic and benthic primary production to benthic organisms has not changed much following dreissenid establishment, the absolute importance of both increased considerably in the post-dreissenid littoral zone: the large increase in invertebrate biomass that followed dreissenid establishment means that the amount of both pelagic and benthic primary production needed to sustain post-dreissenid organisms had to increase considerably. The results of this chapter suggest that dreissenids increase the availability to food to littoral organisms by redirecting pelagic primary production to the benthos and by stimulating littoral benthic primary production. The impacts of dreissenids on littoral benthic organisms probably have large effects on littoral and pelagic fish communities of lakes. Dreissenid mussels translocate biological production to the benthos by stimulating benthic primary production through nutrient excretion and increases in water clarity, by increasing habitat availability for benthic organisms and by biodepositing pelagic material that becomes available to benthic organisms and the fish that feed on them. I argue that hydrodynamic factors are important in controlling the strength of the dreissenid-mediated pelagic-littoral connection in lakes. Because hydrodynamics relate to lake size, a relationship between lake size and the ability of dreissenids to translocate production the littoral zone can be postulated, where dreissenid effects are maximal in intermediate-sized lakes.