Coastal Hypoxia and the Importance of Benthic Macrofauna Communities for Ecosystem Functioning

Abstract

Coastal ecosystems are important because of the vital ecosystem functions and services they provide, but many are threatened by eutrophication and hypoxia. This results in loss of biodiversity and subsequent changes in ecosystem functioning. Consequently, the need for empirical field studies regarding biodiversity-ecosystem functioning in coastal areas has been emphasized. The present field study quantified the links between benthic macrofaunal communities (abundance, biomass and species richness), sediment oxygen consumption and solute fluxes (NO3-+ NO2-, NH4 + , PO4 3-, SiO4, Fe, Mn) along a 7.5-km natural gradient of seasonal hypoxia in the coastal northern Baltic Sea. Sampling was done in late August 2010 in the middle archipelago zone of the Hanko peninsula, Finland. As predicted the macrofaunal communities were decimated with increasing hypoxia, and the nutrient transformation processes were changed at the sediment-water interface, with notably higher effluxes of phosphate and ammonium from the sediment. Solute fluxes varied even during normoxia, which implies a high context-dependency, and could be explained by even small variations in environmental variables such as organic matter and C/N-ratios. Importantly, the low diversity benthic macrofaunal communities, which were dominated by Macoma balthica and the invasive Marenzelleria spp., had a large influence on the solute fluxes, especially under normoxia, but also under hypoxia.

Legacy Effects of Eutrophication on Modern Methane Dynamics in a Boreal Estuary

“…Our results show strong evidence of bioirrigation, i.e., the introduction of oxic bottom water via animal burrows down to 10 cm or more in the sediment, which greatly reduces sediment flux of methane. Indeed, site J was recently shown to have an active community of benthic fauna that affects solute fluxes seasonally (Kauppi et al 2018), whereas in comparison at site L, there is no benthic fauna, likely due to recurrent seasonal hypoxia or anoxia (e.g., Gammal et al 2017 report 0.0 mg/L O 2 at this site for August 2010). We suggest that this is reflected in the porewater profiles, with CH 4 found much closer to the sediment surface at the hypoxic site L due to the absence of bioirrigation.…”

Estuarine Macrofauna Affects Benthic Biogeochemistry in a Hypertrophic Lagoon

“…While in most deep aquatic ecosystems, meiofauna and microbial communities are drivers of benthic processes, in coastal estuarine systems macrofauna play a major role in organic matter mineralization and nutrient cycling [8,[58][59][60]. The analysis of macrofauna diversity, abundance, functional role, and distribution is therefore central to understand coastal lagoon functioning [16,24]. The latter can be defined as the capacity of sediments to process organic matter inputs, avoiding excess carbon accumulation and resulting in fast nutrient turnover.…”

“…Short-term experiments with reconstructed sediments therefore cannot fully reproduce what happens in situ, since the development of bacteria communities along burrows may take weeks and may undergo variations along the life cycle of burrowers [23]. To overcome such limitations, an alternative approach is to collect and incubate undisturbed cores with natural abundance and composition of macrofauna [24][25][26]. A large number of replicate cores can be incubated and sieved at the end of measurements in order to retrieve macrofauna and analyze relationships among macrofauna and biogeochemical processes a posteriori.…”

Unnatural hypoxic regimes

“…Recovery of fauna typically takes longer than recovery of chemical sediment dynamics. Recovery in response to hypoxia caused by organic enrichment can take much longer than that caused by nutrient pollution alone because redox potentials change and undegraded organic matter accumulates in sediment (Gammal et al 2017). Recovery of benthic fauna from hypoxia (<2 mg/L O 2 ) can take months to years (Rosenberg et al 2002, Van Colen et al 2010, Steckbauer et al 2011).…”