A long-term study on crustacean plankton of a shallow tropical lake: the role of invertebrate predation

<p>The primary factor that governs the size and species composition of zooplankton is still a controversial issue and temperature is considered the main factor responsible for latitudinal differences. In waters with a narrow temperature range, such as in the tropics, predation may be a more important factor. Nearly three decades of intermittent studies of the crustacean plankton in a shallow tropical lake revealed that the main event that led to their restructuring was the appearance of a second predator, the water mite <em>Krendowskia </em>sp. The new predator and larvae of the dipteran <em>Chaoborus brasiliensis</em> Theobald exerted a combined, although asymmetrical effect on microcrustaceans. The period when the mite was detected was followed by the restructuring of the crustacean plankton community. Predation by these two invertebrates emerged as the factor responsible for community changes, involving an increased contribution of copepods and decreases in the relative abundance of smaller cladoceran species. In the short term, the mite caused a decrease in species richness and the annual mean instantaneous composition of cladocerans, a predominance of large-sized species (<em>Daphnia ambigua </em>Scourfield<em> </em>and <em>Daphnia gessneri</em> Herbst) and the virtual disappearance of small species (e.g., <em>Bosmina tubicen</em> Brehm). The long-term impact resulted in increased species richness and the dominance of large and medium-sized cladocerans, such as <em>D. gessneri</em> and <em>Ceriodaphnia richardi</em> Sars. The larger body size of three cladocerans, the two <em>Daphnia</em> species and <em>B. tubicen</em>, in the long term, may be a response to the dominant predator, <em>Chaoborus</em>. The seasonal variation in the predator abundance, mainly <em>Chaoborus </em>larvae, allowed the prey to recover during the cool season. The copepods <em>Tropocyclops prasinus meridionalis</em> (Fischer) and <em>Thermocyclops decipiens</em> Kiefer were less affected by predation than the cladocerans; their contribution to the crustacean plankton increased 12-28% after the mite appeared. The top-down effect on crustacean plankton did not affect the phytoplankton, which remained dominated by chlorophyceans and the nanoplankton fraction before and after the zooplankton community restructuring.</p>

Shifts between gelatinous and crustacean plankton in a coastal upwelling region

Bode, A., Álvarez-Ossorio M. T., Miranda, A., and Ruiz-Villarreal, M. 2013. Shifts between gelatinous and crustacean plankton in a coastal upwelling region. – ICES Journal of Marine Science, 70: 934–942. Variability in the dominance of copepods vs. gelatinous plankton was analysed using monthly time-series covering the last 55 years and related to changes in climatic, oceanographic, and fishery conditions in the upwelling region of Galicia (NW Spain). Seasonality was generally the main component of variability in all groups, both along the coast and in the nearby ocean, but no common long-term trend was found. Coastal copepods increased since the early 1990s, and gelatinous plankton increased in the ocean during the 1980s. Different trends were found for gelatinous plankton in two coastal sites, characterized by increases in either medusae or tunicates. In all series, multiyear periods of relative dominance of gelatinous vs. copepod plankton were evident. In general, copepod periods were observed in positive phases of the main modes of regional climatic variability. Conversely, gelatinous periods occurred during negative climatic phases. However, the low correlations between gelatinous plankton and climatic, oceanographic, or fishery variables suggest that local factors play a major role in their proliferations.

Crustacean plankton abundance in the Danube River and in its side arms in Hungary

The spatial distribution and seasonal dynamics of the crustacean zooplankton were studied in the Danube River and in its side arms near Budapest, Hungary. Microcrustaceans were sampled biweekly from October 2006 to November 2007 at eleven sites. Thermocyclops crassus, Moina micrura and Bosmina longirostris added up to 57.6% of the total density. Comparisons of the different water bodies stressed the separation of the eupotamal and parapotamal side arms. Densities in the side arms were one respectively two orders of magnitude higher as compared to the main channel, which was relatively poor in plankton. There were remarkable longitudinal and transversal variations in the abundance of the major zooplankton groups (cladocerans, adult copepods, copepodites, nauplii) and dominant species (t-test, P < 0.05). However, no general pattern was observed, the spatial distribution depended on the examined objects. There were statistically significant seasonal differences in zooplankton abundance (Tukey-test, P < 0.05). Water residence time and water discharge were not found to be related to zooplankton abundance, but water temperature was positively correlated with microcrustacean density.