Nitrogen Fixed By Cyanobacteria Is Utilized By Deposit-Feeders

Abstract

Benthic communities below the photic zone depend for food on allochthonous organic matter derived from seasonal phytoplankton blooms. In the Baltic Sea, the spring diatom bloom is considered the most important input of organic matter, whereas the contribution of the summer bloom dominated by diazotrophic cyanobacteria is less understood. The possible increase in cyanobacteria blooms as a consequence of eutrophication and climate change calls for evaluation of cyanobacteria effects on benthic community functioning and productivity. Here, we examine utilization of cyanobacterial nitrogen by deposit-feeding benthic macrofauna following a cyanobacteria bloom at three stations during two consecutive years and link these changes to isotopic niche and variations in body condition (assayed as C:N ratio) of the animals. Since nitrogen-fixing cyanobacteria have δ15N close to -2‰, we expected the δ15N in the deposit-feeders to decrease after the bloom if their assimilation of cyanobacteria-derived nitrogen was substantial. We also expected the settled cyanobacteria with their associated microheterotrophic community and relatively high nitrogen content to increase the isotopic niche area, trophic diversity and dietary divergence between individuals (estimated as the nearest neighbour distance) in the benthic fauna after the bloom. The three surface-feeding species (Monoporeia affinis, Macoma balthica and Marenzelleria arctia) showed significantly lower δ15N values after the bloom, while the sub-surface feeder Pontoporeia femorata did not. The effect of the bloom on isotopic niche varied greatly between stations; populations which increased niche area after the bloom had better body condition than populations with reduced niche, regardless of species. Thus, cyanobacterial nitrogen is efficiently integrated into the benthic food webs in the Baltic, with likely consequences for their functioning, secondary production, transfer efficiency, trophic interactions, and intra- and interspecific competition.

Feeding strategies of co-occurring suspension feeders in an oligotrophic environment

“…It could also explain the generalized pattern of low δ 15 N values recorded in the Bay of Marseille, notably for zooplankton (Bănaru et al, 2014;Espinasse et al, 2014;Tiano et al, 2014). In the Baltic Sea, previous results confirmed that diazotrophic bacteria can be a significant food source for the whole trophic network (zooplankton and benthic invertebrates), since it can represent up to 80 % of the diet of some species (Karlson et al, 2014;Lesutienė et al, 2014). Assessing accurately the contribution of this source to the diet of suspension feeders, for example through their integration as an endmember in a mixing model, is nevertheless complex since it would require an accurate determination of δ 13 C value for diazotrophic bacteria in the bay of Marseille.…”

“…A predominant integration of bacterial-derived OM could be another explanation for these rather low values. Diazotrophic bacteria have δ 15 N ratios close to 0 ‰ or even negative (Karlson et al, 2014;Briand et al, 2015) and are predominant in two terrigenous water inputs in the bay of Marseille (Huveaune River and wastewater outfall), with the low isotopic ratios of the wastewater outfall consistent with the predominance of diazotrophic bacteria (Bănaru et al, 2014). They are also abundant during summer blooms in the bay of Marseille (2 to 5 10 2 cells mL -1 , Le…”

“…Its retention-system based on a mucus-net mesh is the most efficient to retain particles as small as 0.5 µm (Flood and Fiala-Médioni, 1982;Riisgård and Larsen, 2010;Jumars et al, 2015). Bacteria might represent the missing end-member to complete the mixing-model for this species and are considered as a potential food source for bivalves (Xu and Yang, 2007;Kharlamenko et al, 2008;Pernet et al, 2012;Karlson et al, 2014) and for ascidians (Lesser and Slattery, 2015;Ribes et al, 1998). It could also explain the generalized pattern of low δ 15 N values recorded in the Bay of Marseille, notably for zooplankton (Bănaru et al, 2014;Espinasse et al, 2014;Tiano et al, 2014).…”

Nitrogen fixation by cyanobacteria stimulates production in Baltic food webs

“…Evidence is, however, accumulating that these cyanobacteria are eaten by many grazers, suspension-feeders, and deposit-feeders, often with beneficial effects on their growth and reproduction. In line with this, field studies show that organic matter and bioavailable nitrogen produced by these cyanobacteria are incorporated by pelagic, littoral, and benthic food webs (e.g., Rolff 2000 ; Karlson et al 2014 ; Lesutiene et al 2014 ), challenging the view that cyanobacteria do not contribute to secondary production. Moreover, grazing on toxic cyanobacteria is species-specific and influenced by many factors (e.g., toxin concentrations and availability of other prey), with concomitant feed-back effects on toxin production (Gorokhova and Engström-Öst 2009 ; Engström-Öst et al 2011 ).…”

Developing a lake management strategy by dovetailing lake monitoring with paleolimnological techniques: a case study from a kettle lake on the Oak Ridges Moraine (Ontario, Canada)

“…Although the peak value we recorded (δ 15 N = 4%) is lower than normally recorded values, the δ 15 N does record a steady increase over time, as expected. Results can be skewed by cyanobacteria that alter isotope ratios; for example, δ 15 N values can be depleted after cyanobacterial blooms due to uptake of diastrophic 14 N. In a study from the Baltic Sea, Karlson et al (2014) found that sediment δ 15 N values were depleted when compared to sediment values prior to a cyanobacterial bloom.…”