Interactions between ecosystem engineering by burrowing sandprawns (Callichirus kraussi) and nutrients: consequences for benthic community structure and ecosystem functioning
Doctoral Thesis
2019
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Abstract
Non-trophic interactions are significant structuring agents of ecological communities. Knowledge of how this process drives ecosystem functioning and community structure either individually, or interactively with other processes, is however, limited, particularly in shallow soft- littoral ecosystems. At a local level, such systems are dominated by allogenic engineers such as the burrowing axiid sandprawn Callichirus kraussi Stebbing, which has important effects on macro- and meiobenthic assemblages. C. kraussi is distributed across the South African coastline, from the subtropical Mozambican border to the temperate west coast borders with Namibia. Bioturbation — the principal mechanism by which C. kraussi engineers influence associated biota in sedimentary systems, also has significant effects on sediment properties, biofilms, microalgal and microbial composition. However, theory suggests that ecosystem engineering effects are contextually dependent and contingent upon processes that are temporally and spatially variable. In South Africa, variations in background nutrient levels across the coastline is significant, with the West coast being dominated strong upwelling, which increases biological productivity. Within the west coast, upwelling is also seasonally variable, being strongest in summer. In view of this natural variability in nutrient levels across the coast, seasonal variability within the west coast and the fact that the sandprawn C. kraussi dominates across these conditions, the central focus of this PhD thesis was to investigate how ecosystem engineering by sandprawns C. kraussi and nutrient levels individually or interactively influence assemblages and ecological processes in coastal soft-sediment ecosystems. The study was carried out in Langebaan Lagoon, which is a marine lagoonal system on the west coast of South Africa that is dominated by sandprawns and subjected to seasonal upwelling that is a feature of the west coast. This thesis was based on the two principal approaches, viz. a field comparative study and in situ experiments. The field observational study investigated the responses of macroand meiofaunal communities to sandprawn bioturbation impacts between upwelling and non-upwelling seasons, with the aim of understanding how upwelling nutrient pulses modify these benthic assemblages. Benthic and water column chlorophyll-a (chl-a) levels were also measured, while meiofaunal communities were investigated within sandprawn burrows (burrow-walls) and at the sediment surface. It was hypothesized that chl-a levels, and community metrics would be lower in winter (non-upwelling) but increase in summer upwelling season due to nutrient pulses. It was also hypothesized that community metrics would be negatively correlated with sandprawn density due to bioturbatory effects (sediment turnover) in winter (non-upwelling) but this relationship would become neutral or positive increased in summer, due to increased productivity offsetting negative bioturbative effects. Clear, non-intuitive and ecologically interesting outcomes emerged from the field study. Firstly, benthic chl-a concentrations appeared to be lower in summer relative to winter in 2015, although, this pattern disappeared in 2016. In contrast, water column chl-a concentrations within the lagoon channel conformed to the posed hypothesis, being consistently greater in summer relative to winter. Even though both macro- and meiofaunal assemblages differed significantly between seasons, the hypothesis that community metrics would be greater in summer relative to winter season was not overwhelmingly supported by these findings. Pearson correlation analyses revealed that sandprawn effects were generally weaker within seasons but stronger when winter and summer data were combined and investigated per year. In terms of the latter, most community metrics and benthic chl-a levels were generally negatively correlated with sandprawn abundances, but this varied seasonally and spatially. Overall, results of the comparative study suggest that increases in water column chl-a levels do not necessarily translate into increases in benthic chl-a and community metric levels. One possible reason for this is that increasing bioturbation by C. kraussi overrides nutrient enrichment effects. This conclusion though is spatially dependent, given that results of correlation analyses were site-specific. The factorial field experiment employed in this dissertation manipulated nutrients (fertilizer capsules, Plantacote N: P: K) and sandprawns (C. kraussi densities) to investigate their individual and interactive effects on benthic assemblages. I hypothesized within the context of the grazer-reversal hypothesis of Proulx and Mazumder (1998) that, responses of diversity metrics should alter from a linear decrease at the ambient nutrient level to a unimodal hump-shaped response at the intermediate nutrient level and then to a linear increase at highest nutrients. Generally, emerging patterns for meiofaunal diversity metrics and individual morphotypes were inconsistent with the prediction of the grazer-reversal model. Instead, responses were of an increasing and decreasing nature. The outcomes of this investigation, however, revealed that meiofaunal community structure at the sediment surface was significantly affected by the main effect of sites and sandprawn densities, while within burrow-walls, the interaction between sites and nutrients, but also sandprawn densities alone, significantly affected meiofaunal community structure. Overall, meiofaunal results of this investigation showed limited support for the grazer-reversal hypothesis. Although the findings of this investigation also indicated limited support for the grazer-reversal hypothesis on the macrofauna community, interactive effects of nutrient enrichment and sandprawn densities emerged to influence the macrofaunal community structure and abundances of certain individual species/taxa. Generally, macrofaunal community metrics exhibited both increasing and decreasing patterns in response to experimental treatments, however, in some instances unimodal hump-shaped patterns emerged. There was evidence of macrofaunal functional groups (i.e. suspension feeders) conforming to the hypothesis posed, but this was site-specific. Overall, I conclude that an interplay between nutrients and sandprawns does not exert strong influences on the benthic communities of Langebaan Lagoon. However, sandprawn ecosystem engineering, mainly in the form of bioturbation overrides nutrient enrichment effects in regulating benthic assemblages.
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Qwabe, W. 2019. Interactions between ecosystem engineering by burrowing sandprawns (Callichirus kraussi) and nutrients: consequences for benthic community structure and ecosystem functioning.