Intertidal estimates of sea urchin abundance reveal congruence in spatial structure for a guild of consumers.

We hypothesized congruence in the spatial structure of abundance data sampled across multiple scales for an ecological guild of consumers that exploit similar nutritional and habitat resources. We tested this hypothesis on the spatial organization of abundance of an herbivorous guild of sea urchins. We also examined whether the amount of local along-shore rocky habitat can explain the observed spatial patterns of abundance. Standardized estimates of abundance of four intertidal sea urchins-Diadema cf. savignyi, Echinometra mathaei, Parechinus angulosus, and Stomopneustes variolaris-were determined by six observers at 105 sites across 2,850 km of coast of South Africa. For each species and observer, wavelet analysis was used on abundance estimates, after controlling for potential biases, to examine their spatial structure. The relationship between local sea urchin abundance and the amount of upstream and downstream rocky habitat, as defined by the prevailing ocean current, was also investigated. All species exhibited robust structure at scales of 75-220 km, despite variability among observers. Less robust structure in the abundances of three species was detected at larger scales of 430-898 km. Abundance estimates of sympatric populations of two species (D. cf. savignyi and E. mathaei) were positively correlated with the amount of rocky habitat upstream of the site, suggesting that upstream populations act as larval sources across a wide range of scales. No relationship between abundance and habitat size was found for P. angulosus or S. variolaris. Within the range of scales examined, we found robust congruence in spatial structure in abundance at the lower, but not the larger, range of scales for all four species. The relationship between abundance and upstream habitat availability in two species suggests that larval supply from upstream populations was probably the mechanism linking habitat size and abundance.

Zooplankton grazing pressure is insufficient for primary producer control under elevated warming and nutrient levels.

Within a given ecosystem, species persistence is driven by responses to the effects of biotic and abiotic stressors. Ongoing climatic shifts and increased pollution pressure have created the need to assess potential effects and interactions of physical and biotic factors on coastal ecosystem processes to project ecosystem resilience and persistence. In coastal marine environments, primary production dynamics are driven by the interaction between bottom-up abiotic effects and biotic effects induced by top-down trophic control. Given the many environmental and climatic changes observed throughout coastal regions, we assessed the effects of interactions among temperature, nutrients and grazing in a laboratory-based microcosm experiment. We did this by comparing chlorophyll-a (chl-a) concentrations at two temperatures in combination with four nutrient regimes. To test for subsequent cascading effects on higher trophic levels, we also measured grazing and growth rates of the calanoid copepod Pseudodiaptomus hessei. We observed different phytoplankton and zooplankton responses to temperature (17 °C, 24 °C) and nutrients (nitrogen only (N), phosphates only (P), nitrogen and phosphates combined (NP), no nutrient additions (C)). Contributions of predictors to model fit in the boosted regression trees model were phosphates (42.7%), copepods (23.8%), nitrates (17.5%) and temperature (15.9%), suggesting phosphates were an important driver for the high chl-a concentrations observed. There was an increase in total phytoplankton biomass across both temperatures, while nutrient addition affected the phytoplankton size structure prior to grazing irrespective of temperature. Phytoplankton biomass was highest in the NP treatment followed by the N treatment. However, the phytoplankton size structure differed between temperatures, with microphytoplankton being dominant at 24 °C, while nanophytoplankton dominated at 17 °C. The P and C treatments exhibited the lowest phytoplankton biomass. Copepod abundances and growth rates were higher at 17 °C than at 24 °C. This study highlights that bottom-up positive effects in one trophic level do not always positively cascade into another trophic level. It was, however, evident that temperature was a limiting factor for plankton abundance, productivity and size structure only when nutrients were limiting, with top-down pressure exhibiting minimal effects on the phytoplankton.