In shallow floodplain ponds, we previously found that fall zooplankton abundances were highest overall in the more permanent ponds. However, for more temporary ponds, crustacean zooplankton were much more abundant in years when these ponds dried out in late summer/early fall. Our previous work also found that total summer-fall precipitation, while highly variable among years, has increased significantly over the past 6.5 decades. This study focused on two ponds in years with below-average and above-average June-November rainfall, resulting in strong inter-year differences in hydroperiod for each pond. These ponds represent the deeper, more permanent ponds (Pond 5) and the shallower, more temporary ponds (Pond 1) of this floodplain. The deeper ponds are more open and the shallow ponds are forested. The central question of this study asked if inter-year or inter-pond differences in resources could explain the observed differences in fall cladoceran abundance. We assessed individual body length and clutch size, population egg production, and population size structure for two relatively common cladoceran genera (Daphnia, Simocephalus). Both taxa have a relatively large maximum body size and potential clutch size.
Results/Conclusions
Size-specific clutch size (SSCS) of Simocephalus varied between years and pond types. SSCS was lower in Pond 5 in the wetter year (40% vs. 2012), despite comparable abundance between years (Sep-Nov). Egg production in Pond 1 was not detectable in October 2013 due to very low animal abundance. By December, however, as abundance increased, SSCS values in Pond 1 were ca.highest values observed so far (max. 11 eggs per 1.5 mm ind.). In October 2012, Pond 1 abundance was 20X higher vs. 2013, and egg production was measurable, but SSCS was 25% lower in Pond 1 than in Pond 5. Overall, trends in SSCS were not consistent with trends in population abundance. Food resources for zooplankton assemblages of ponds appeared to vary with hydrologic pattern. In the drier year, findings support hypotheses related to more quantity and/or higher quality diet in the deeper, more permanent pond vs. the temporary pond. Both ponds showed better food resources overall vs. the wetter year, when resource concentrations may have been more limiting to cladoceran recruitment. In the temporary pond, 2013 inundation was correlated with especially low population abundances and egg counts until early December. Findings suggest fundamental differences in the dynamics of resource environments for the shallow, forested ponds and the deeper, more open ponds of this floodplain landscape.