Towards predicting basin-wide invertebrate organic biomass and production in marine sediments from a coastal sea

Abstract

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.

Figure 1. Sampling region (Strait of Georgia) showing general areas and number of benthic invertebrate samples along with bottom bathymetry based on multibeam data (a) (courtesy of Natural Resources Canada).

The indicated boundary between the Northern and Southern Straits relates to the limitations of influence to sediments from the Fraser River discharge; and b) Core and sediment trap locations for organic flux measurements (see Table S2 for sources and dates of cores).

Figure 2. Geographic distribution of habitat variables for the Strait of Georgia.

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.

Figure 3. Geographic distribution of mean body mass (kj) per organism in the Strait of Georgia.

Figure 4. 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).

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 Table S4).

Figure 5. Geographic distribution in the Strait of Georgia of mean total invertebrate production/biomass (P/B) ratio, and values relative to modified organic carbon flux (N = 987), depth and percent sand (N = 1067).

Mean values for each sample location and time are shown on figures for visual simplicity and are included in Table S5. Note that the multi-factor regressions (described in results) used only data points for which all three environmental factors were available (N = 987).

Figure 6. Geographic distribution in the Strait of Georgia of mean total invertebrate biomass, and values relative to modified organic carbon flux (N = 987), depth and percent sand (N = 1067).

Mean values for each sample location and time are shown on figures for visual simplicity and are included in Table S5. 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.

Figure 7. 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).

Mean values for each sample location and time are shown on figures for visual simplicity and are included in Table S5. 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.