stable isotope data for deposit-feeders

Rawdata for each species, station and year: the d15N and d13C data (d13C is corrected for CN ratio according to Post et al. 2007 Oecologia) as well as CN rati

Nitrogen Fixed By Cyanobacteria Is Utilized By Deposit-Feeders

<div><p>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 (<i>Monoporeia affinis, Macoma balthica</i> and <i>Marenzelleria arctia</i>) showed significantly lower δ¹⁵N values after the bloom, while the sub-surface feeder <i>Pontoporeia femorata</i> 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.</p></div

GLM results for δ¹⁵N and δ¹³C for all species.

<p>Only winning models according to AIC criteria are shown. The reference category for the estimate for bloom is the pre-bloom; negative values denote a decrease after bloom. The reference station is stn Uttervik.</p

Sampling occasions in relation bloom.

<p>Cyanobacteria bloom development in 2009 and 2010 at stn B1 (near sampling stations Håldämman and Uttervik), dotted line, and stn H3 (close to stn Mörkö), solid line. The X-axis is scaled for Julian days and benthos sampling dates are indicated by arrows. See text for the description of bloom composition. In 2010, stn Mörkö was sampled only in late June and September.</p

Carbon and nitrogen content pre- and post-bloom.

<p>Carbon and nitrogen (%) in the test species (left to right: Mac - <i>M. balthica</i>, Mz - <i>M. arctia</i>, Mon - <i>M. affinis</i> and Pon - <i>P. femorata</i>) before (white) and after (grey) the bloom. Values are median with 25 and 75% percentiles as well as 95% CI (n = 6 for Mac and Mz, n = 5 for Mon and n = 3 for Pon).</p

Isotopic niche for each species and the entire community pre- and post-bloom.

<p>Stable isotope bi-plots illustrating the isotopic niche of the four species, <i>M. affinis</i> (black), <i>M. arctia</i> (red), <i>M. balthica</i> (green), <i>P. femorata</i> (blue) at three study sites (top stn Håldämman, mid stn Uttervik and bottom stn Mörkö) before and after the bloom, for both years (2009, left panels, and 2010, right panel). The dotted lines enclose convex hull area for each species and solid line shows the standard ellipse area, SEAc, for the entire community. Note the presence of <i>M. affinis</i> post-bloom at stn Mörkö 2009 inflates the niche area, since it was not found pre-bloom. Excluding <i>M. affinis</i> at this station in 2009 results in a reduced niche area (as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0104460#pone-0104460-g005" target="_blank">Fig 5</a>, top panel).</p

Relationship between bloom intensity and bloom induced change in C:N and isotopic niche.

<p>Concurrent changes (post-bloom/pre-bloom) in C:N ratio and SEAc in relation to the bloom intensity (area under curve; top and mid panels) and to each other (bottom panel). Each data point represents one species, station and year.</p

Sediment element composition and isotope ratio before (B) and after (A) the bloom.

<p>Values are means based on 3–6 analytical replicates per station and time point. Precision was <0.1% for C and N and <0.1‰ for δ¹³C and δ¹⁵N.</p

Cross-species overlap in isotopic niche pre- and post-bloom.

<p>Isotopic niche overlap defined as an area common for the two populations in relation to the total isotopic space occupied by these populations (proportion; left y-axis) and total community niche (arbitrary units; right y-axis) calculated using pre-bloom and post-bloom datasets for each station and year (see also <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0104460#pone-0104460-g004" target="_blank">Fig. 4</a> for raw data). The grey bars show the overlap estimate for each pair of species whose niches were found to overlap, white bars are the total community niche, and pie charts on the top of the gray bars show overlapping proportions of the isotopic niche for each population in question. Datasets that have no overlap during the study period are not included.</p

Number of individuals for each species (Mac = <i>Macoma balthica</i>, Mz = <i>Marenzelleria arctia</i>, Mon = <i>Monoporeia affinis</i> and Pon = <i>Pontoporeia femorata</i>), station and year, used for stable isotope analysis.

<p>B denotes samples taken before the cyanobacteria bloom, A those after the bloom; zero - not found.</p