Size structure of marine soft-bottom macrobenthic communities across natural habitat gradients: implications for productivity and ecosystem function

Abstract

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(-6)J to 2(16)J) 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/m(2)/yr/δ(15)N); (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/m(2)/yr/δ(15)N) 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/m(2)/yr/δ(15)N). 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.

Figure 1. Map of general study locations of macrobenthic grab sampling surveys compiled in the Strait of Georgia database (coastal British Columbia, Canada).

Circles indicate locations of grab samples. Line indicates demarcation between Northern and Southern Strait of Georgia.

Figure 2. Response of size structure to water depth (m) categories.

(A) Cluster analyses show relationships among depth categories based on their community size structure. SIGTREE analyses assess which relationships are statistically significant (asterisks indicate rejection of the null hypothesis (at α = 0.01) that the two groups being linked are homogeneous. (B) Cumulative biomass size spectra for each depth category show how biomass accumulates across size categories of macrobenthic organisms (based on log₂ organic biomass).

Figure 3. Response of size structure to sediment percent fines categories.

(A) Cluster analyses show relationships among sediment percent fines categories based on their community size structure. SIGTREE analyses assess which relationships are statistically significant (asterisks indicate rejection of the null hypothesis (at α = 0.01) that the two groups being linked are homogeneous. (B) Cumulative biomass size spectra for each percent fines category show how biomass accumulates across size categories of macrobenthic organisms (based on log₂ organic biomass).

Figure 4. Response of size structure to modified (by quality, δ¹⁵N) organic flux categories (gC/m²/yr/δ¹⁵N).

(A) Cluster analyses show relationships among modified organic flux 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) Cumulative biomass size spectra for each modified organic flux category show how biomass accumulates across size categories of macrobenthic organisms (based on log₂ organic biomass).

Figure 5. Response of size structure to combined habitat factors.

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. (A) 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) 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.