Stuart F. Mitchell

An empirical model for sediment resuspension in shallow lakes

Suspended solids concentrations were measured at routine 2–3 week intervals and on additional windy days for at least one year in each of seven shallow (mean depth < 2 m) south Island, New Zealand lakes. Surface wave characteristics were estimated from water depths and local meteorological data using a shallow-water wave forecasting model for fetch-limited waves. Bottom shear stresses were computed from surface wave characteristics for the sampling stations and for a hypothetical lake-average station. The calculated shear stresses were, on average, much better predictors of suspended solids concentrations than alternative models using two different functions of wind speed, wave height2/depth or wavelength/depth. A combination of the sample station and lake average shear stresses provided slightly better predictions than the sample station values alone, suggesting that currents also contribute significantly to the concentration at a given point. Regressions of suspended solids on the combined functions had r2 values ranging from 0.74–0.73 in the seven lakes. The slopes of these regressions were negatively related to the settling velocity of the lowest quartile of the sediment, and to macrophyte biomass, in multiple regression (r2 = 0.94, p < 0.01).

Sub-fossils of cladocerans in the surface sediment of 135 lakes as proxies for community structure of zooplankton, fish abundance and lake temperature

To elucidate the possibilities of using zooplankton remains in the surface sediment to describe present-days community structure and population dynamics of zooplankton, fish abundance and temperature, we compared contemporary data sampled in the pelagial during summer with the sediment record from the upper 1 cm of the sediment in 135 lakes covering a latitude gradient from Greenland in the north to New Zealand in the south. The abundance of three genera Bosmina, Daphnia and Ceriodaphnia of the total pool of ephippia was significantly related to the total abundance of the same taxa in the pelagic zone. However, in most lakes the abundance of Ceriodaphnia was higher in the sediment than in the water, which may be attributed to the overall preference by this genus for the littoral habitat. Using contemporary data from 27 Danish lakes sampled fortnightly during summer for 10 years, we found substantial inter-annual variations in the abundance of Daphnia spp., Ceriodaphnia spp., B. longirostris and B. coregoni. Yet, the sediment record mimicked the medium level well for most of the lakes, which suggests that the sediment record provides an integrated picture of the pelagic cladoceran community, which otherwise can be obtained only by long-term frequent contemporary sampling for several years. The contribution of Daphnia to the sum of Daphnia and Bosmina ephippia was negatively correlated with the abundance of fish expressed as catch per night in multi-mesh sized gill nets (CPUE). Yet, region-specific differences occurred, which partly could be eliminated by including nutrient state expressed as total phosphorus (TP) in a multiple regression. The average ratio of ephippia to the sum of ephippia and carapaces of Bosmina varied 40-fold between the sampling regions and was significantly negatively related to summer mean air temperature, and for Danish lakes also, albeit weakly, to fish CPUE but not to chlorophyll a. Apparently, temperature is the most important factor determining the ratio of parthenogenetic to ephippia producing specimens of Bosmina. We conclude that the sediment record of cladocerans is a useful indicator of community structure of pelagic cladocerans and the abundance of fish and temperature.

Reconstructing the historical changes in Daphnia mean size and planktivorous fish abundance in lakes from the size of Daphnia ephippia in the sediment

The zooplankton community structure in lakes is highly influenced by size-selective predation by fish, with small zooplankton species dominating at high predation pressure. Remains of cladocerans are preserved in the sediment and may be used to trace historical changes in fish predation. We determined how contemporary data on planktivorous fish were related to the size of Daphnia ephippia (dorsal length) in the surface sediment (0-1 cm) of 52 mainly shallow lakes with contrasting densities of fish and nutrients (TP: 0.002-0.60 mg P l-1). Density of fish expressed as catch per unit effort, in terms of numbers in multiple mesh-sized gill nets (CPUEn), decreased significantly with increasing mean size of ephippia. The relationship was improved by adding TP as an independent variable, now explaining 90% of the variation in CPUEn on the full data set covering lakes in Denmark, Greenland and New Zealand, and 78% if only data on Danish lakes were used. CPUE by weight of planktivorous fish and mean weight of Daphnia in the pelagial during summer were also related to ephippial size. By including contemporary data on established relationships between the sizes of egg-bearing female Daphnia and ephippia, we inferred changes in the CPUEn, mean size of ephippia-bearing Daphnia and summer mean body weight of Daphnia from ephippial size in four lakes during the past 1-2 centuries. In a hypertrophic lake subject to periodic fish kills, Daphnia mean body weight was high and CPUEn was low compared with those in two eutrophic lakes, while CPUEn was low and Daphnia body weight was high in the least eutrophic, clearwater lake. Estimated CPUEn and Daphnia mean weight in the surface sediment of these four lakes corresponded well with contemporary data. Only small changes in ephippial size with time were observed in the clearwater lake and in one of the lakes that had suffered early eutrophication, while major changes occurred in the two other lakes that had been subjected to a major increase in nutrient input or fish kills. We conclude that Daphnia ephippia preserved in the surface sediments of lakes may be a useful and efficient method to quantify the present-day abundance of planktivorous fish and Daphnia mean size. The method is particularly valid in surveys aimed to give a general picture of the fish stock and the ecological state in a set of lakes in a region rather than a precise estimate for a single lake. Though some evidence is provided, more work is needed to evaluate whether the equations are valid for hind-casting in down-core palaeoecological studies.

Primary production in a shallow eutrophic lake dominated alternately by phytoplankton and by submerged macrophytes

Abstract Tomahawk Lagoon No. 2, a shallow eutrophic New Zealand lake, has shown irregular cyclic variations between phytoplankton and macrophyte dominance since 1963. During a 4-year study, which encompassed a change from phytoplankton to macrophyte dominance, there was a large decline in phytoplankton biomass and production, and an increase in macrophyte biomass. However, there was little increase in the macrophyte production, and the total primary production of the lake declined sharply, by a factor of 4. The effects of phytoplankton having better access to the available light and a longer growing season, and the more rapid nutrient cycling expected among planktonic communities, clearly outweighed those of macrophytes having access to sediment nutrients in this lake. Grazing by black swans was calculated to have removed about 15–25% of the production of macrophytes during their growing season, or 20–50% per annum.

Do planktivorous fish structure the zooplankton communities in New Zealand lakes

Abstract We critically evaluate the prevailing view that the species composition of zooplankton communities in New Zealand lakes, and the abundance and size distribution of the zooplankton, are more commonly controlled by food and temperature than by predation. We conclude that predation may play a far more important role than was hitherto thought. This conclusion is based first on the seasonal zooplankton dynamics of several New Zealand lakes being similar to those in Danish lakes, in which fish predation has been shown to have major effects. Second, the indigenous planktivorous fish fauna in New Zealand lakes is dominated by larvae of benthic fish with a long breeding season (bullies) and other small potent zooplanktivores such as smelt and larvae of galaxiids. Patterns of diurnal vertical migration of zooplankton in several New Zealand lakes during summer are also consistent with it being a mechanism to escape from predation. Third, the dominant piscivorous fish (brown trout and rainbow trout) are only...

Effects of riparian willow trees (Salix fragilis) on macroinvertebrate densities in two small Central Otago, New Zealand, streams

Abstract The effects of an introduced willow tree species (Salix fragilis) on the densities of macroinvertebrates were examined in two Central Otago, New Zealand, streams during 1991. Significantly lower invertebrate densities and biomass were observed in willow‐lined sections of the streams than in nearby open sections in summer, autumn, and winter. This result was observed in riffles and pools, for most dominant species and nearly all functional feeding groups. The effect was not associated with differences in the amount of fine paniculate organic matter ( 5 mm) were significantly higher in willow‐shaded riffles, but this did not result in increased abundance or biomass of shredders. Willow trees reduced incident stream illumination by as much as 80%, but did not appear to influence water chemistry between open and willow‐shaded s...

Seasonal succession and vertical distribution of zooplankton in Lake Hayes and Lake Johnson

Seasonal variations in the species composition, abundance, biomass, and vertical distribution of zooplankton in two eutrophic, warm‐monomictic lakes are described for the period December 1969 to February 1972. In Lake Hayes, Boeckella dilatata was the dominant crustacean and Ceriodaphnia dubia was subdominant. Lake Johnson was dominated by Ceriodaphnia and Bosmina meridionalis and Boeckella were subdominant. Daphnia were common in spring in Lake Hayes, but very rare in Lake Johnson. Chydorus and Asplanchna appeared in summer in both lakes. Zooplankton were slightly more abundant in both lakes in 1971 than in 1970; the annual mean was 1.7 times higher in Lake Johnson than in Lake Hayes in both years. Biomass was highest in spring. In Lake Hayes it ranged from 0.7 to 4.6 g.m‐2 dry weight; in Lake Johnson it fluctuated in 1971 from 0.3 g.m‐2 in March to 7.4 g.m‐2 in November. Animals occurred at all depths during holomixis, but were nearer the surface during stratification. Vertical distribution was often bi...

Seasonal succession and vertical distribution of phytoplankton in Lake Hayes and Lake Johnson, South Island, New Zealand

Abstract In two small monomictic lakes near Queenstown, South Island, New Zealand, algal associations characteristic of eutrophic waters were present throughout most of the period from December 1969 to February 1972. In Lake Hayes, five of the six algal genera which were recorded in the plankton in 1948–50 were still present, but Anabaena appeared to have increased. Blooms of A. flos‐aquae which were not recorded 20 y ago (Jolly 1952) occurred in the lake in the three summers of this study and in winter of the second year. Melosira granulata dominated the plankton in winter; at other times of the year Closterium aciculare, Cyclotella kuetzingiana, and Staurastrum spp. were dominant. In Lake Johnson the major algae differed from one year to another. Blooms of Anabaena flos‐aquae formed during the first two summers but were absent in the third, when Closterium aculum var. variabile was dominant. Peridinium cinctum was abundant throughout the first year, especially in December 1969 and October 1970, when con...

Diel variation in horizontal distribution of Daphnia and Ceriodaphnia in oligotrophic and mesotrophic lakes with contrasting fish densities

Recent studies document diel horizontal migration by large zooplankton in eutrophic shallow lakes. Risk of predation from planktivorous fishes could induce such behaviour. We studied diel horizontal distribution of cladocerans in 31 mainly shallow oligotrophic and mesotrophic New Zealand (NZ) and North American (NA) temperate lakes. In terms of weight, fish catch per net (CPUE w ) in multiple mesh-sized gill nets was similar in the two sets of lakes, while CPUE by number (CPUE n ) was overall higher in the NA lakes. Unlike previous results from eutrophic, temperate lakes, we found no significant diel variations in density in the pelagic and littoral zones, suggesting no diel horizontal migration of zooplankton. In the NZ lakes, Daphnia and Ceriodaphnia were evenly distributed between the littoral zone and the pelagial, while in the NA lakes Daphnia were more abundant in the pelagial and Ceriodaphnia in the littoral zone. In the oligotrophic fishless NZ lakes, large Daphnia carinata dominated, whereas the smaller Ceriodaphnia dubia dominated in lakes with high CPUE’s. In both the NZ and the NA lakes, Daphnia showed no clear correlation to fish CPUE n . However, in the NA lakes, Daphnia occurred at fish CPUE n values at which they were eliminated in the NZ lakes, which may be related to differences in water transparency, reflecting a higher chlorophyll a and humic content in the NA lakes.

Grazing by black swans (Cygnus atratus Latham), physical factors, and the growth and loss of aquatic vegetation in a shallow lake

Abstract Hawksbury Lagoon, a shallow coastal New Zealand lake, alternates between a clear-water, macrophyte-dominated state, and a turbid, phytoplankton-dominated state. The potential role of black swans ( Cygnus atratus Latham) in stabilising the phytoplankton dominated state by grazing on macrophytes was investigated during a period of increase and decline in the benthic vegetation ( Nitella ) in 1993–1994. The swan population density was closely correlated with plant biomass ( r 2 = 0.95). Although the swan population became as high as 25 ha −1 direct grazing consumption was slight. The grazing rate was 0.007 day −1 , by comparison with plant growth rates of 0.06–0.10 day −1 , and loss rates in periods of decline of 0.07–0.18 day −1 . Indirect effects of the swans on the plants through nutrient recycling and bioturbation, are also unlikely to have been important. Concentrations of suspended solids and phytoplankton, and light attenuation, remained high throughout the study. Plant biomass normally increased when the benthic photon irradiance exceeded 7% of that at the surface, and decreased when it was lower than that. We conclude that lack of light was far more important than swan grazing for plant decline. When light or other conditions for macrophyte growth are marginal, the cumulative effect of waterfowl grazing consumption might well be critical, however, for keeping macrophyte biomass below the threshold for macrophyte dominance, in spite of the consumption being small.