Lake Ecology

Lake biological communities in New Zealand and Australia appear to be structured quite differently to those of Northern Hemisphere lakes. At the base of the aquatic food chain the paradigm of nutrient limitation by phosphorus cannot be generalized to lakes in the Southern Hemisphere. Nutrient limitation by nitrogen has often been observed in open waters of many New Zealand lakes, particularly those of the Central Volcanic Plateau region of the North Island. Relatively recent land development and intensification of farming activities present a significant challenge for managing lakes in this region due to increased nitrogen leaching to the groundwater aquifers that contribute most of the lakes' nutrient load.

Phytoplankton succession in the temperate lakes follows the typical succession from diatoms and green algae to cyanobacteria and motile algae as turbulent mixing decreases. In Northern Australia the cyanobacteria can dominate lake ecosystems for much of the year. Cylindrospermopsis is notable as a nitrogen-fixing cyanobacterium that has relatively recently become widespread in both Australia and New Zealand, having mainly dominated in (sub) tropical systems in the past.

In the littoral region native submerged macrophyte communities tend to be low-growing turf communities that are particularly susceptible to being out-competed by some of the taller canopy-forming exotic species such as Lagarosiphon major and Egeria densa, with the latter species able to still be highly productive under nutrient-enriched conditions. Only a very limited number of macroinvertebrate species in New Zealand can actually consume macrophytes directly, so again there is high susceptibility to invasions by the tall, canopy-forming species which are grazed minimally and contribute little to carbon cycling through the aquatic food chain.

Susceptibility to eutrophication may also be significantly enhanced in New Zealand and Australia, compared with the Northern Hemisphere, as the zooplankton communities are considered to be depauperate, without many of the large planktivorous species that are characteristic of North Hemisphere lakes. Furthermore there are few if any obligate planktivor-ous fish species that may exert direct control by grazing on phytoplankton populations, though some of the smaller native fish species (e.g., bullies and kaoro) have planktivorous larval stages. Iconic sal-monids (e.g., brown trout and rainbow trout in particular) and perch have been extensively introduced into New Zealand and the southern temperate region of Australia and likely play a major role in top-down control of zooplankton populations, thereby influencing trophic structure and once again weakening the capacity for zooplankton to exert top-down control on phytoplankton populations. Introductions of coarse fish (tench, rudd, catfish, and koi carp), mostly liberated and spread illegally, have also been linked to declining water quality in lakes in the Auckland and Waikato regions of New Zealand.

For shallow lakes there is, however, a strong similarity between Australasia and the Northern Hemisphere related to the existence of alternative stable states; a clear-water, macrophyte dominated phase and a turbid, phytoplankton dominated phase. In both cases nutrient supply may play an important role in effecting switches from one state to another, but bio-manipulations involving fish removal in some Northern Hemisphere lakes have also been highly effective in restoring a clear-water phase. It is less likely that biomanipulation would meet with the same degree of success in counterpart Australasian lakes under the reduced top-down structuring, and physical factors such as strong winds in the prevailing maritime climate of New Zealand and coastal Australia and anthropogenic salinization in Australia, as well as the ephemeral nature of many Australian inland waters, greatly complicate application of alternative stable state theory to shallow lakes of Australia and New Zealand. Examples of lakes that have shown alternative stable states in Australia include; Lake Mokoan (Victoria), Little White Lake (Western Australia), Crescent (Tasmania), Sorrell (Tasmania) and in New Zealand; Ellesmere (South Island), Horowhenua (North Island), Omapere (North Island) and Waikare (North Island). All of these lakes are characterized by switches to the turbid state except Little White Lake, which appears to be transitional, in which the transitional state appears to be regulated by changes in salinity.

The Australian saline lakes typically have fairly low species richness but can have high productivity.

Resistant propagules of invertebrates, plants and phytoplankton can rapidly germinate upon flooding. The salt-tolerant charophyte Lamprothamnium papulosum and the aquatic angiosperms Ruppia sp. and Leilaena tend to dominate the macrophyte community in the saline lakes. In Australia, most of the taxa found in saline lakes are endemic and typically species richness decreases with increasing salinity. Lake Corangamite (Victoria, Australia) has suffered from diversion of the major freshwater tributary from the lake and subsequently has experienced steadily increasing salinity, to the point where it hosts very few aquatic species and supports a fraction of the bird life that it did historically.

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