Towards Predicting Basin-Wide Invertebrate Organic Biomass and Production in Marine Sediments from a Coastal Sea

<div><p>Detailed knowledge of environmental conditions is required to understand faunal production in coastal seas with topographic and hydrographic complexity. We test the hypothesis that organic biomass and production of subtidal sediment invertebrates throughout the Strait of Georgia, west coast of Canada, can be predicted by depth, substrate type and organic flux modified to reflect lability and age of material. A basin-wide database of biological, geochemical and flux data was analysed using an empirical production/biomass (P/B) model to test this hypothesis. This analysis is unique in the spatial extent and detail of P/B and concurrent environmental measurements over a temperate coastal region. Modified organic flux was the most important predictor of organic biomass and production. Depth and substrate type were secondary modifiers. Between 69–74% of variability in biomass and production could be explained by the combined environmental factors. Organisms <1 mm were important contributors to biomass and production primarily in shallow, sandy sediments, where high P/B values were found despite low organic flux. Low biomass, production, and P/B values were found in the deep, northern basin and mainland fjords, which had silty sediments, low organic flux, low biomass of organisms <1 mm, and dominance by large, slow-growing macrofauna. In the highest organic flux and biomass areas near the Fraser River discharge, production did not increase beyond moderate flux levels. Although highly productive, this area had low P/B. Clearly, food input is insufficient to explain the complex patterns in faunal production revealed here. Additional environmental factors (depth, substrate type and unmeasured factors) are important modifiers of these patterns. Potential reasons for the above patterns are explored, along with a discussion of unmeasured factors possibly responsible for unexplained (30%) variance in biomass and production. We now have the tools for basin-wide first-order estimates of sediment invertebrate production.</p> </div

Size Structure of Marine Soft-Bottom Macrobenthic Communities across Natural Habitat Gradients: Implications for Productivity and Ecosystem Function

<div><p>Size distributions of biotic assemblages are important modifiers of productivity and function in marine sediments. We investigated the distribution of proportional organic biomass among logarithmic size classes (2⁻⁶J to 2¹⁶J) in the soft-bottom macrofaunal communities of the Strait of Georgia, Salish Sea on the west coast of Canada. The study examines how size structure is influenced by 3 fundamental habitat descriptors: depth, sediment percent fines, and organic flux (modified by quality). These habitat variables are uncorrelated in this hydrographically diverse area, thus we examine their effects in combination and separately. Cluster analyses and cumulative biomass size spectra reveal clear and significant responses to each separate habitat variable. When combined, habitat factors result in three distinct assemblages: (1) communities with a high proportion of biomass in small organisms, typical of shallow areas (<10 m) with coarse sediments (<10% fines) and low accumulation of organic material (<3.0 gC/m²/yr/δ¹⁵N); (2) communities with high proportion of biomass in the largest organisms found in the Strait, typical of deep, fine sediments with high modified organic flux (>3 g C/m²/yr/δ¹⁵N) from the Fraser River; and (3) communities with biomass dominated by moderately large organisms, but lacking the smallest and largest size classes, typical of deep, fine sediments experiencing low modified organic flux (<3.0 gC/m²/yr/δ¹⁵N). The remaining assemblages had intermediate habitat types and size structures. Sediment percent fines and flux appear to elicit threshold responses in size structure, whereas depth has the most linear influence on community size structure. The ecological implications of size structure in the Strait of Georgia relative to environmental conditions, secondary production and sediment bioturbation are discussed.</p> </div

Geographic distribution in the Strait of Georgia of mean total invertebrate production, and values relative to sample modified organic carbon flux (N = 987), depth and percent sand (N = 1067).

<p>Mean values for each sample location and time are shown on figures for visual simplicity and are included in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0040295#pone.0040295.s006" target="_blank">Table S5</a>. Note that the multi-factor regressions (described in results) used only data points for which all three environmental factors were available (N = 987). The overlapping red triangles represent samples from near the Fraser River discharge.</p

Geographic distribution of habitat variables for the Strait of Georgia.

<p>These include a) sediment % sand, and b) modified organic carbon flux (organic carbon flux/del 15N) including values measured from ₂₁₀Pb dated cores as well as extrapolated values for locations with biological samples lacking concurrent core data.</p

Results of BEST (BIO-ENV) routine (PRIMER-E, PlymouthMarine Laboratory) based on Spearman’s correlation between size structure resemblance matrix (Bray-Curtis similarity, standardized organic biomass data)and normalized habitat matrix (Euclidean distance).

<p>Data are otherwise not transformed.</p

Response of size structure to combined habitat factors.

<p>Samples were re-categorized based on previous cluster analyses into shallow (<10 m) and deep (≥10 m), Coarse sediments (<10% fines) and Fine sediments (≥10% fines), and low organic flux (<3 gC/m²/yr/δ¹⁵N) and high organic flux (≥3 gC/m²/yr/δ¹⁵N) and analyzed together. (<b>A</b>) Cluster analyses show relationships among habitat categories based on their community size structure. SIGTREE analyses assess which relationships are statistically significant. Asterisks indicate p<0.0001, (and thus rejection of the null hypothesis at α = 0.01 that the two groups being linked are homogeneous). P-values >0.01 are indicated above nodes. (<b>B</b>) Cumulative biomass size spectra for each habitat category show how proportional biomass accumulates across size categories of macrobenthic organisms (based on log₂ organic biomass). Horizontal lines are placed at 95% of total biomass and 50% of total pooled biomass.</p

Distribution of the proportion of total invertebrate organic biomass and production contributed by small faunal (<1 mm) organisms, relative to % sand, depth and modified organic flux (N = 65).

<p>Only % sand was significantly related to either biomass or production (r² shown on plots, p<0.01; regression coefficients described in results and data shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0040295#pone.0040295.s005" target="_blank">Table S4</a>).</p

Examples of faunal production estimates from the literature, including habitat type and faunal groups analysed, illustrating ranges measured for infauna from different habitats using different estimation methods (see Cusson and Bourget [9] for a recent and more thorough global review).

++<p>classical direct measurements (respiration/cohort biomass).</p>*<p>Empirical models such as Brey <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0040295#pone.0040295-Brey1" target="_blank">[7]</a>.</p>∧<p>Phylogenetic P/B conversions from literature.</p>∧∧3<p>H]-leucine incorporation.</p