Zooplankton Length Research Articles

Role of predation in biological communities in naturally eutrophic sub-Arctic Lake Mývatn, Iceland

Effects of fish predation on consumers tend to be particularly strong in oligotrophic Arctic and sub-Arctic lakes. However, it remains unclear whether the fish influence the trophic structure and dynamics of naturally eutrophic lakes in these cold environments with simple food web structures. To study this, we conducted a 3-month in situ-controlled experiment in sub-Arctic Lake Mývatn, Iceland. We used the planktivorous fish three-spined sticklebacks (Gasterosteus aculeatus) as the main top predator. The cladocerans Eurycercus lamellatus and Acroperus harpae were significantly associated with fishless enclosures, whereas the rotifers Polyarthra sp. and Filinia terminalis were significantly associated with the fish enclosures. Fish predation led to a significant increase in phytoplankton biomass and a reduction in the zooplankton:phytoplankton biomass ratio, the mean zooplankton length and cladoceran mean biomass. Fish effects might have been stronger if a bloom of Anabaena in August had not overridden potential cascading effects. We argue that both top-down and bottom-up forces are important for structuring the communities in the lake. Our results suggest that Arctic and sub-Arctic lakes may undergo important changes in trophic dynamics if they get warmer and more nutrient rich as expected with the global climate change.

Zooplankton and the total phosphorus – chlorophyll a relationship: hierarchical Bayesian analysis of measurement error

Zooplankton grazing is important in resolving residual variation around the total phosphorus – chlorophyll a relationship. In empirical studies, zooplankton body size is often a better predictor of residual variation than zooplankton biomass. We investigate whether higher measurement error associated with zooplankton biomass may explain its lower predictive ability. We collected five replicate zooplankton biomass samples in 19 lakes, allowing us to quantify measurement error in volumetric zooplankton biomass with greater precision than in previous studies. A hierarchical Bayesian model was used to assess the predictive ability of volumetric zooplankton biomass and mean individual zooplankton length, corrected for measurement error. We found consistent effects of total zooplankton biomass, but not zooplankton length, on chlorophyll a. This finding does not appear to be related to the higher precision with which total zooplankton biomass was measured in our study, but rather to ecological factors. Interlake variation outweighed the effects of measurement error in estimating the strength of relationships between zooplankton variables and chlorophyll a. Our findings therefore suggest that studies to estimate zooplankton effects on phytoplankton should allocate resources to study a larger range of lakes over different time periods than to process replicate samples to reduce measurement error.

Seasonal Zooplankton Abundance and Size Fluctuations across Spatial Scales in Lake Pend Oreille, ldaho

ABSTRACT We studied seasonal zooplankton community dynamics along spatial scales in Lake Pend Oreille, Idaho in 1997–1998 to determine if zooplankton community changes brought about by establishment of Mysis relicta in the 1970's persist today, and to relate zooplankton length and abundance fluctuations to zooplanktivory. Long-term trends in zooplankton species composition and relative abundance, as well as seasonal patterns of species succession are apparently unchanged since M. relicta establishment. Cladoceran zooplankters became abundant earlier in the spring and remained abundant later into autumn in 1998 than in 1997. Fluctuations in zooplankton lengths and abundance were more pronounced along horizontal gradients extending from littoral to pelagic zones than along vertical gradients extending downward from the surface, and these fluctuations were more pronounced in 1998 than in 1997. Zooplankton length variability along spatial scales occurred most often in late spring/early summer, and in autumn. The trends we observed are consistent with prior speculation that zooplanktivory by M. relicta structures the Lake Pend Oreille zooplankton community, although abiotic factors may also explain our results.

Aquatic Invertebrate Assemblages in Shallow Prairie Lakes: Fish and Environmental Influences

ABSTRACT We sampled zooplankton and benthic macroinvertebrate assemblages in 30 shallow natural lakes to determine the effects of the environment (i.e., habitat and fish abundance) on invertebrates. Zooplankters were identified to genus, and up to 120 individuals per genus were measured. Macroinvertebrates were identified to order, class, or family. Fish communities were also sampled. Relative abundances of zooplankton and macroinvertebrates were low at increased chlorophyll a concentrations, although mean zooplankton length increased with total phosphorus, possibly because of an increased proportion of microzooplankton (rotifers and copepod nauplii) at higher phosphorus levels. Canonical correspondence analysis revealed that zooplankton and macroinvertebrate abundance was influenced by submersed vegetation coverage, whereas zooplankton abundance and size structure were also related to productivity (i.e., chlorophyll a and total phosphorus). However, relative abundance of fish species or fish feeding guilds was not strongly correlated with zooplankton or macroinvertebrate abundance or zooplankton size structure. Physical habitat (e.g., vegetation coverage) may exert substantial influences on invertebrate assemblages in these lakes, possibly providing a refuge from fish predation.

Potential Effect of Invasion by an Exotic Zooplankter, Daphnia Iumholtzi

ABSTRACT We observed an increasing population of Daphnia lumholtzi in Lake Springfield, II, near the northern limit of its known distribution, from 1992 through 1994. The Daphnia lumholtzi population peaked with water surface temperature. Since the establishment of D. lumholtzi, the composition of spring peak crustacean zooplankton has shifted from >75% cladocerans to >75% copepod nauplii. There was also a corresponding decrease in crustacean zooplankton length from 0.41 ± 0.04 in 1992 to 0.18 ± 0.01 in 1994 during peak abundance. Since D. lumholtzi does not appear until summer, direct competition with native zooplankton would not account for this change in spring zooplankton community structure. We propose two indirect mechanisms by which the presence of D. lumholtzi may lead to a decrease in the numbers of native zooplankton overwintering as adults, thus impacting the spring community. Late summer competition among zooplankton species may reduce native zooplankton populations. Also, selective avoidance of D. lumholtzi by fish predators may increase predation pressure on native zooplankton.

Counting and identifying copepod species with an in situ electronic zooplankton counter

The potential for identifying and separating copepod species and stages with an in situ electronic zooplankton counter, which was developed and tested as a biological tool, has been examined. The instrument was mounted on the Batfish, a depth-controllable towed body equipped to measure chlorophyll a, temperature, salinity, and depth. Zooplankton length and diameter distributions measured by the zooplankton counter were compared with measurements made using a microscope. Detailed analysis of the data shows that the electronic counter has sufficient resolution to separate and identify dominant species of copepods consisting of Clausocalanus arcuicornis, Metridia lucens, Calanus finmarchicus, and Euchirella rostrata sampled from the Scotian Shelf, and Centropages brachiatus, Calanus chiliensis, and Eucalanus inermis sampled from the Peruvian Shelf. The examples of Batfish data included show the separation of dominant species and copepod abundance estimates from the Peruvian Shelf. The relative merits of the electronic measurements and microscope analysis are discussed.