Carolyn W. Burns

Biological drivers of zooplankton patchiness

Until recently, biological drivers of plankton aggregation were under-appreciated, because most studies concentrated on physical processes. New technological advances, novel experiments and theory have shifted focus to the pivotal role of behaviour in plankton patch dynamics. Our review highlights four biological drivers of zooplankton spatial patchiness and brings together recent research on well studied marine and freshwater taxa, primarily copepods and cladocerans. Diverse and powerful behavioural responses by zooplankton to physical and chemical signals are shown to contribute to the formation and breakdown of zooplankton patches over several different spatial scales.

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 the epizoic ciliate, Epistylis daphniae, on growth, reproduction and mortality of Boeckella triarticulata (Thomson) (Copepoda: Calanoida)

The transmission of Epistylis daphniae infection among Boeckella triarticulata and the effect of peritrich epizoites on the copepod hosts were examined at 15 °C in the laboratory. After being paired with carrier copepodites, 97.5% of non-carrier adults became hosts to E. daphniae. When food was present there was no difference between carriers and non-carriers in growth of copepodites, and in survival and reproduction of adults. When food was absent, copepods infected with E. daphniae survived for shorter periods than non-carriers (p < 0.002). Therefore, colonial peritrichs may contribute to the decline of copepod populations when they are in a food-limited environment.

Linking land-use, water body type and water quality in southern New Zealand

Land-use and vegetation cover have been linked to the nutrient levels (nitrogen, phosphorus) of surface waters in several countries. However, the links generally relate to streams and rivers, or to specific types of standing water, for example shallow lakes in a geologically defined region. We measured physical variables and nutrient chemistry of 45 water bodies representative of the wide range of lentic wetland environments (swamps, riverine wetlands, estuaries, reservoirs, shallow lakes, deep lakes) in Otago, New Zealand, and related these to catchment variables and land-use in order to assess the potential influence of catchment modification on water quality of these diverse wetlands. Catchment boundaries and land cover were derived from maps using ArcView GIS software. Our predictions that concentrations of nutrients and other components of water quality would correlate positively with the nature and intensity of catchment modification were confirmed in multivariate analyses. Physical and chemical measures were positively related to the extent of modification in the catchment (percentage of the catchment in pasture, planted forest, scrub and urban areas), and negatively related to lack of catchment modification (more of the catchment in bare ground, tussock grassland and indigenous forest). The strong negative correlations between nutrient concentrations, suspended sediment, water colour and the percentage of tussock cover in the catchment imply that increased conversion of the␣native tussock grassland to pastoral farming in␣Otago will increase nutrient concentrations and␣reduce water quality of the diverse lentic ecosystems.

LIFE HISTORY CONSEQUENCES OF FOOD QUALITY IN THE FRESHWATER COPEPOD BOECKELLA TRIARTICULATA

Food quality often has profound effects on life history traits and individual fitness, altering rates of growth and development, changing the timing of reproduction, and shifting the trade-off between egg size and egg number. Few data are available on the effects of food quality on copepod life history traits. We measured several life history traits on a large number of individuals to document the effects of food quality on individual traits, on life history correlates, and on a composite measure of individual fitness in the freshwater copepod Boeckella triarticulata Thomson. Nauplii were raised individually on two diets: one consisted of the high quality alga Cryptomonas sp. (abbreviated as CR), and the second diet consisted of a combination of Cryptomonas sp. and the low quality cya- nobacterium Anabaena flos-aquae (CA). The mixed CA diet slowed growth and develop- ment so that individuals raised on this diet were older and smaller at metamorphosis and maturity. Despite these effects, there were no differences between diets in survival to maturity, and male copepods raised on the mixed diet lived significantly longer than females or than either sex raised on Cryptomonas alone. Females raised on the mixed diet produced more and larger clutches than those raised on CR, so that total egg production increased on this diet, although large intradiet variation obscured statistical differences between diets in these parameters. Intradiet variation was due to a large range in the number of clutches produced by individual females: some individuals produced 10-15 clutches, contradicting previous descriptions of this species as semelparous. Although diet affected age at first reproduction, it had no significant effect on individual fitness, estimated as X. Boeckella triarticulata achieved high fitness either by minimizing age at first reproduction (CR diet) or by increasing reproductive output (CA diet). Data collected from cast-off exuviae allow nondestructive measures of individual life history traits along with a composite measure of individual fitness. Combining these two analyses is an important step in unraveling life history correlates and in identifying the selective forces driving life history evolution in these crustaceans.

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...

The effects of temperature and food on copepodite development, growth and reproduction in three species of Boeckella (Copepoda; Calanoida)

Three species of Boeckella (B. triarticulata, B. dilatata and B. hamata) were reared from copepodite I to adult at three naturally fluctuating food levels and three temperatures in a 3 × 3 × 3 factorial design. Development times, lengths, times to produce the first clutch, clutch sizes, egg voumes, interclutch times, infertile clutches, total egg production, total reproductive time and mortality were measured. Development times for the three species were surprisingly similar but mortality differed among the treatments. The three species differed in their reproductive strategies. B. triarticulata, the largest species, produced a few large clutches. B. dilatata, the smallest species, produced many small clutches and invested the most energy per egg. B. hamata produced larger clutches than B. dilatata but not as many. B. hamata was the most tolerant of low, food levels and exhibited the most plasticity across the treatments. Thus, greater plasticity in life history characters is associated with survival in a wider-range of conditions.

Some observations on the diet of the backswimmer, Anisops wakefieldi (Hemiptera: Notonectidae)

In a series of laboratory experiments, A. wakefieldi was presented with a variety of prey from different systems to investigate its potential for cannibalism and for eating cladocerans, copepods and rotifers. Large (ca. 7 mm) Anisops decreased the survivorship of small (ca. 3 mm) Anisops, but ate only 0.1 individual predator-1 d-1. Ingestion rates of small Anisops on small (<0.5 mm) and large (0.5–0.9 mm) Ceriodaphnia dubia were 53 and 28 individuals predator-1 d-1, while those of large Anisops on these prey were about twice as high. Clearance rates showed that small Anisops ate small and large C. dubia with similar efficiencies, while large Anisops ate large C. dubia much more efficiently. Anisops ate C. dubia much more efficiently than the calanoid copepod Boeckella hamata. Only the large Anisops ate adult B. hamata. Large Anisops had a very low ingestion rate (0.6 individuals predator-1 d-1) on the larger congener, B. triarticulata. Small, but not large, Anisops ate the rotifer Synchaeta pectinata and small Anisops presented with a natural assemblage of rotifers ate S. pectinata and Polyarthra dolichoptera, but not four other species with a more well-developed lorica – Anuraeopsis fissa, Brachionus angularis, Keratella cochlearis and K. slacki. Ingestion rates on Synchaeta and Polyarthra were 14–20 individuals Anisops-1 d-1.

Insights into zooplankton‐cyanobacteria interactions derived from enclosure studies

Abstract Few workers have used enclosures specifically to study interactions between zoo‐plankton and cyanobacteria. Differences among studies in enclosure size, nutrient level, plankton abundance and species composition, presence or absence of fish, and length of experiments make generalisations difficult. Zooplankton had no direct effect on the growth of ungrazed cyanobacteria (Anabaena flos‐aquae, Aphanizomenon flos‐aquae, large Microcystis colonies) in short‐term ( 1 month) enclosure studies. When large cyanobacteria were abundant, some Daphnia spp. snowed reduced reproduction and development. When large grazers were abundant they suppressed the growth of edible, colonial cyanobacteria (Aphanizomenon elenkinii, small Microcystis colonies). By altering the ambient light and nutrient environment, large zooplankton may suppress cyanobacteria; evidence for the importance of grazers in promoting cyanobacterial dominance by removing competing phytoplankton species is equivocal. Zoop...

Responses of lake phytoplankton to micronutrient enrichment: a study in two New Zealand lakes and an analysis of published data

Abstract.Natural and anthropogenic changes in nutrient concentrations can affect phytoplankton in marine and freshwater environments. However, potential micronutrient limitation of phytoplankton productivity in fresh waters is often overlooked. To investigate the responses of lake phytoplankton to micronutrient enrichment, we conducted a study in two contrasting New Zealand lakes, and analysed data from the published international literature.We undertook nutrient enrichment bioassays of phytoplankton communities sampled from a mesotrophic reservoir and an eutrophic coastal lake to determine the relative occurrence of micronutrient (iron, boron, cobalt, copper, molybdenum) and macronutrient (nitrogen, phosphorus) limitation. In the mesotrophic reservoir, phytoplankton productivity was phosphorus limited. No evidence of micronutrient limitation was found in six bioassays in summer. In the eutrophic lake, tenfold enrichment of ambient micronutrient concentrations increased the primary productivity in four of 11 bioassays. During a cyanobacterial bloom in the eutrophic lake, experimental enrichment with boron, cobalt, copper or molybdenum increased primary productivity by 40%. These four micronutrients are commonly applied as agricultural fertiliser in the lake’s catchment. Nitrogen or phosphorus enrichment had no effect on phytoplankton productivity at this time. Micronutrient limitation has been reported in more than 40 lakes internationally, and our analysis of published data suggests that the prevalence of micronutrient limitation is unrelated to lake size or trophic state. As micronutrient enrichment can significantly increase phytoplankton productivity in a range of lakes types, the potential contribution of micronutrient enrichment to eutrophication should not be overlooked.