Canyon conditions impact carbon flows in food webs of three sections of the Nazaré canyon
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- 3.2 Network indices
Results3.1 Food web structureThe models of the upper and middle canyon could be solved with the default equality and inequality constraints. However, the first attempt to solve the model of the lower section with the default set of constraints was unsuccessful, which indicates that some of the data embedded in the linear inverse model are in conflict with each other. Subsequent analysis showed that the minimum degradation of semi-labile detritus (4761 · 8.21·10-4 = 3.9 mmol C m-2 d-1, Table 1 & 2) was higher than the maximum rates of total carbon oxidation and carbon deposition (0.90 and 1.3 mmol C m-2 d-1, respectively). Since the latter two data are site-specific field data, it was decided to modify the literature bound on the minimum rate of semi-labile degradation through pre-multiplication with the temperature limitation factor (Tlim = 0.30, Table 2). This allowed the model to be solved and its implications will be discussed below. The mean flow values and standard deviations for the three sections of the Nazaré canyon are reported in Web appendix 1. The quality of the model solutions was evaluated with the Coefficient of Variation (CoV), which is the standard deviation of a flow divided by the mean flow value. As such, the CoV provides an indication for the residual uncertainty in the solution, where flows with a relatively large residual uncertainty have a comparatively high CoV and flows with a relatively small residual uncertainty have a comparatively low CoV. All flows in all three canyon sections had a CoV that was smaller than 1. Maximum CoV were 0.86, 0.90 and 0.86 for the upper, middle and lower canyon section, respectively and were associated with transfer of one the nematode compartments to the (surface) deposit-feeding macrobenthos. The CoV was smaller than 0.75 for 81%, 73% and 82% of the flows of the upper, middle and lower canyon section, respectively, and the CoV was smaller than 0.50 for 40%, 40% and 45% of the flows. Total carbon input (mmol C m-2 d-1) to the different food webs was 7.98±0.84 (5% labile, 75% semi-labile and 20% refractory detritus), 9.30±0.71 (9% labile, 89% semi-labile and 2% refractory detritus) and 1.26±0.03 (6% labile, 90% semi-labile and 4% refractory detritus) for the upper, middle and lower canyon section, respectively. Total respiration was 4.52±0.28, 5.06±0.30 and 0.86±0.02 mmol C m-2 d-1 and organic carbon burial was 3.05±0.80, 3.85±0.35 and 0.34±0.04 mmol C m-2 d-1 for the upper, middle and lower canyon section, respectively. Prokaryotes dominated carbon respiration in the upper (70%) and lower (82%) section, but their contribution to total respiration is lower (38%) than the total megafaunal respiration in the middle section (57%) (Table 5). Summed meiofaunal respiration contributes 21% tot total respiration in the upper, 3% in the middle and 13% in the lower canyon section, whereas summed macrofaunal respiration contributes 8% in the upper, 1% in the middle and 5% in the lower section. Summed export fluxes (i.e. secondary production not consumed within the food web) differed between the sections with 0.18±0.08, 0.10±0.05 and 0.02±0.006 mmol C m-2 d-1 for the upper, middle and lower section, respectively. The structural differences between the food webs become apparent when flows are plotted as mean net values in a circular food web structure (Fig. 1). The main differences between the upper and lower section are the more important role of the non-selective feeding meiofauna compartment (Fig. 1A vs. 1C) and MacPS compartment (Fig. 1D vs 1F) in carbon cycling in the upper canyon section. Of similar importance, however, is the pathway of deposition of semi-labile, dissolution to dissolved organic carbon, prokaryotic uptake of this DOC and prokaryotic respiration in the upper and lower sections (Fig. 1A vs. 1C). Consistent with their comparatively low contribution to total respiration, the carbon flows related to the macrofaunal compartments are small, except for the MacPS compartment in the upper canyon section that show up mostly in the lower row of Fig.1. The food web structure of the middle canyon section stands out primarily because of the dominant role of the MegDF and, to a lesser extent, MegSDF compartments (Fig. 1B and 1H). Moreover, carbon cycling by the macrobenthic compartments, especially MacPS, is less important as compared to the upper and lower canyon section. There is a dominance of semi-labile detritus in the diets of most faunal compartments in the upper section of the Nazaré canyon, with semi-labile detritus supplying between 53% and 95% of carbon of the non-predatory compartments and 11-12% of the predatory compartments MeiPS and MacPS, respectively (Fig. 2A). Labile detritus (2 – 15%) and prokaryotes (2 – 22%) supply a comparable lower fraction of carbon to the non-predatory compartments and 4 – 5% to the predatory compartments. Non-predatory meiofaunal compartments fuels the meiofaunal and macrofaunal predatory compartments in similar amounts (21 – 50%). Faunal diets of the non-predatory compartments in the middle section are comparable to the upper section, with a dominance of semi-labile detritus (42 – 93%) and labile (2 – 21%) detritus (Fig. 2B). The diet contribution of prokaryotes to non-predatory faunal compartments varies between 2 and 21%. Dependence on selective and non-selective feeding meiofaunal compartments is highest for predatory meiofauna (80%), followed by predatory macrofauna (48%) and <10% for the other macrofaunal and megafaunal compartments. The diet of the predatory/scavenging macrofaunal compartment is diverse, with no clear dominance of any resource (3 – 25%). The diet compositions in the lower section of the Nazaré canyon resemble overall those of the upper section (Fig. 2A vs. 2C). Again, semi-labile detritus is most important (between 76 – 98%) in the diets of non-predatory faunal compartments. Diet contributions of labile detritus and prokaryotes are similar for selective feeding meiofauna (9-10%), non-selective meiofauna (each 1%), predatory/omnivore meiofaunal (each 5%), surface-deposit feeding macrofauna (each 5%), deposit-feeding macrofauna (each 1%) and predatory/scavenging macrofauna (4-5%) (Fig. 2C). The meiofaunal compartments MeiSF + MeiNF are important resources for the meiofaunal predators/omnivores (together 80% of the diet) and predatory (69%) macrofauna, but are of lesser importance for surface-deposit (10%), deposit feeding (1%). The diet composition of predatory/scavenging macrofauna is diverse though with a high importance of selective feeding meiofauna (54%) and lower contributions ranging from 1 - 11% from other resources. The diet of suspension-feeding macrofauna is similar among the canyon sections and is partitioned among labile (32 – 36%) and semi-labile (64 – 68%) detritus from the water column. The dominant fate of prokaryotic production in all three sections is mortality (52 – 88%) and grazing by meiofauna in the upper canyon section (31%) and by megafauna in the middle section (36%) (Fig. 3A-C). The majority of the meiofaunal secondary production is grazed by macrofauna in the upper (56%) and lower (47%) canyon section, while megafaunal grazing is important in the middle section (36%) and grazing by meiofauna (MeiPO) is important with a consistent contribution of 18 – 23% in the three sections (Fig. 3D-F). The fate of macrofaunal production is partitioned similarly in all three canyon sections with maintenance representing 22 – 24%, mortality 29 – 34%, predation by macrofauna (MacPS) 2 – 20% and export 29 – 42% (Fig. 3G-I). The fate of megafauna is dominated by maintenance respiration (91%) and with limited contributions of mortality (5%) and export (4%) (Fig. 3J). 3.2 Network indicesThe network indices total system throughput (  ), Finn cycling index ( ) and average mutual information ( ) were calculated for the three sections (Fig. 4) and compared (Table 6). The does not differ significantly between the upper and middle sections with median values of 41.1 and 39.7 mmol C m-2 d-1, respectively, but is significantly lower in the lower section with a median of 6.7 mmol C m-2 d-1 (Table 6). Differences in are highly significant between canyon sections (Table 6) and median values are 0.13, 0.06 and 0.17 for the upper, middle and lower section, respectively. is not significantly different between the upper (median of 2.21) and middle (2.22) canyon section, but significantly lower for the lower section (2.12).
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