Bottom‐up control in the benthos of ocean‐exposed sandy beaches?

Abstract

Bottom-up control is a fundamental structuring force in food webs. Food webs of ocean-exposed sandy beaches are predicted to be bottom-up controlled systems, underpinned by imported organic matter rather than in situ primary production.This ecological model of resource-based regulation of biological assemblages is juxtaposed against a prevailing paradigm built around a dominance of physical drivers in sandy beach ecosystems. Surprisingly, given the apparently ubiquitous energetic subsidies of beach food webs, the central premise of bottom-up control has not been tested. Here we experimentally manipulated in situ nutrient levels on a sandy beach to test food web responses at the levels of primary producers (benthic microalgae) and their grazers (meiofauna). The meiofauna community as a whole appeared most strongly influenced by the local physical environment, particularly changes in sediment grain size -this supports the traditional 'environmental control paradigm'. We also detected a significant, positive response of two consumer groups of the meiofauna (nematodes, ostracods) to nutrient enrichment that supports a model of biological, bottom-up control. Although the predicted response of elevated producer biomass following nutrient enrichment was not detected, intense grazing pressure on new, stimulated production may have masked positive responses by the primary producers. Multichannel regulation of food webs is likely for many exposed sandy beaches, albeit an often lower importance of in situ bottom-up forces compared with stronger environmental control.

Benthic microalgae under the influence of beach nourishment in the Gulf of Odessa (the Black Sea)

“…This lies in accordance with the results on other investigations of sand beaches (cahoon et al, 2012;Facca & sFriso, 2007). Sediment composition being one of environmental factors, which determines the community organization on sandy shores, is an important driver of biomass, community composition and production in the microphytobenthos (schLacher & hartWiG, 2013).…”

“…Increase in nutrients is known to strongly influence benthic food webs through the stimulation of primary productivity (schLacher & hartWiG, 2013). Dinophyta and Cyanophyta are disposed to mixotrophy and can be considered as indicators of organic outbreak (aGatZ et al, 1999).…”

“…Environmental control is usually considered to be the main mechanism that structures ecological communities, whereas biological factors have much less of an influence (schLacher & hartWiG, 2013). In this context, the ecological significance of benthic microalgae in sandy beach ecosystems remains poorly resolved.…”

Nonadditive effects of consumption in an intertidal macroinvertebrate community are independent of food availability but driven by complementarity effects

“…Diatom density is a direct result of marine primary production which is an important ecosystem function. Diatom production can be enhanced, for example, by an increase in anthropogenic nutrient input which is currently widespread in marine systems (Allgeier, Rosemond, & Layman, 2011) or reduced by overconsumption (e.g., Schlacher & Hartwig, 2013). Wave and current strength subsequently may affect diatom supply to the beach, creating resource heterogeneity that influences the relation between species composition and community and ecosystem processes (Dyson et.…”

Encounter competition partly explains the segregation of the sandy beach amphipods Bathyporeia pilosa and Bathyporeia sarsi. A mesocosm experiment

“…Moreover, food supply has been shown to be even more important for structuring communities on sandy beaches (Dugan et al, 2003;Lastra et al, 2006;Rodil et al, 2012). Studies on sandy beach food web dynamics indicate that beside allochthonous wrack material (Dugan et al, 2003;Ince et al, 2007;Lastra et al, 2008), also primary production in the water column (Lastra et al, 2006) and on dissipative beaches even in situ primary production (Bergamino et al, 2011;Schlacher & Hartwig, 2012) are supporting the beach food web and are structuring meio-and macrofaunal communities. Yet, biological interactions, in particular competition, are considered of minor importance in structuring benthos zonation patterns (McLachlan, 1983(McLachlan, , 1996(McLachlan, , 2001McLachlan et al, 1993).…”