In tidal, brackish lakes, salinity gradients may strongly influence the abundance and taxonomic composition of aquatic communities, often resulting in transitions from freshwater to euryhaline-marine communities. Zooplankton are an important link in aquatic food webs, as grazers of phytoplankton and as prey for higher trophic levels such as fish. In freshwater/brackish lakes crustaceans are frequently the most visible component of the zooplankton. Consequently, changes in crustacean zooplankton communities may lead to changes in the trophic structure and functioning of aquatic ecosystems.

I investigated the effects of salinity gradients associated with tidal intrusions on the crustacean zooplankton of a tidal coastal lake, Lake Waihola, South Island, New Zealand. To determine the effect of salinity on the crustacean zooplankton community of Lake Waihola, laboratory experiments were used to calculate the salinity tolerance thresholds of each of the dominant taxa. The concentrations of crustacean zooplankton, salinity, chlorophyll a and suspended particulate matter in Lake Waihola were also measured over two years to determine the effects of these tidal intrusions on the lake community.

The crustacean zooplankton community was dominated by two species of calanoid copepod, Boeckella hamata Brehm and Gladioferens pectinatus Brady, and the cladoceran species, Daphnia carinata King. Salinity gradients from 0 – 2000 mg L-1 Cl occurred spatially within Lake Waihola, whereas salinities ranged from < 100 to > 2000 mg L-1 Cl over annual cycles.

In laboratory tests, B. hamata and D. carinata were the least salt tolerant of the dominant taxa, with tolerance thresholds of < 1000 and < 750 mg L-1 Cl, respectively. I demonstrated, however, that B. hamata can produce resting eggs that persist in the sediments of the lake during periods of high salinity and allow this taxon to recolonise the plankton when salinities are low. In contrast, G. pectinatus was the most salt tolerant of these taxa, showing a tolerance of c. 1000 mg L-1 Cl that was increased when exposed to gradual increases in salinity.

In Lake Waihola, variation in salinity explained > 50% of the species-environment correlation over spatial and temporal scales. The abundances of B. hamata and D. carinata were negatively correlated with salinity, whereas that of G. pectinatus was positively correlated. Over the two-year study period increases in salinity resulted in a transition from the predominantly freshwater taxa, B. hamata and D. carinata, to the predominantly estuarine copepod, G. pectinatus.

Tidal current velocities of > 35 cm s-1 in Lake Waihola exceeded the swimming speeds of zooplankters and significantly influenced the distribution of the crustacean zooplankton.

Salinity appears to be the most important variable driving the abundance and taxonomic composition of the crustacean zooplankton community in Lake Waihola, whereas water movement significantly affects the distribution of this community. My findings suggest that increased tidal intrusions into coastal lakes, as a result of increased regulation and abstraction of inflowing rivers, combined with global warming and rising sea levels, may cause significant changes in crustacean zooplankton communities. Such changes may then result in changes to the trophic structure and functioning of tidal coastal systems.