Evidence for long-term seamount-induced chlorophyll enhancements

Seamounts are ubiquitous global features often characterized by biological hotspots of diversity, biomass, and abundance, though the mechanisms responsible are poorly understood. One controversial explanation suggests seamount-induced chlorophyll enhancements (SICE) subsidize seamount ecosystems. Using a decade of satellite chlorophyll data, we report substantial long-term chlorophyll enhancements around 17% of Pacific seamounts and 45% of shallow (< 100 m) seamounts, with the highest probability of detection at shallow, low-latitude seamounts. SICE is shown to enhance chlorophyll concentrations by up to 56% relative to oceanic conditions, and SICE seamounts have two-fold higher fisheries catch relative to non-enhancing seamounts. Therefore, seamount-induced bottom-up trophic subsidies are not rare, occurring most often at shallow, heavily exploited seamounts, suggesting an important subset of seamounts experience fundamentally different trophic dynamics than previously thought.


Fig. S2.
The 10-year average chlorophyll at Seamount Y3681803. This represents the maximum slope found in this dataset between depth and chlorophyll (112% increase from 4000 m to 30 m). Bathymetry is overlain by colors representing decadal per-pixel average chlorophyll-a concentrations in mg/m3. The white target symbol highlights the summit of the main feature of interest. The grey line represents the 30 m contour. Figure and map generated using R 42 version 3.5.1 with packages marmap 41 and lattice 51 .  proportion of pixels of the seamount region with depths shallower than 800 m To allow for complex seamount morphology. Seamount morphology may not be best characterized by summit depth, a single point; 800 m is an approximate daytime depth for the deep scattering layer LT_avgchl long term (10 year) average chlorophyll for the seamount region To allow for differences in background (open-ocean) chlorophyll concentration. SICE may not be noticeable in highly productive regions since the production is already high propZeu proportion of pixels of the seamount region with depths shallower than the average euphotic zone depth of the seamount region for the sample period (one decade) To allow for complex seamount morphology. Seamount morphology may not be best characterized by summit depth, a single point; primary production occurs only in the euphotic zone; if the seamount itself reaches into the euphotic zone physical effects will influence primary producers SST_summit A decadal average sea surface temperature at summit position To link observed effects to physical mechanisms. Temperature influences stratification (determined by temperature, salinity, and pressure), and local density stratification influences the type of physical processes occurring at seamounts including internal wave/tide dynamics and Taylor cone formation.  Table S3.
Model selection criteria and explanations for the logit model for presence or absence of SICE for Allain subset. This table details formulas for each of the "best" AICc equivalent models. Frequency of significance is tabulated for each predictor that was significant in at least one best model (given in number significant/total best models). Finally, the model chosen for drawing plots in Figure 2 is presented.   Table S5.
Historical total family catch from the Watson et al. 2018 database grouped by SICE and Non-SICE seamounts from the Allain seamount subset excluding seamounts with one or more emergent cells in the seamount area (N=177). Families making up more than 1% of the total historical catch at each SICE and Non-SICE seamounts. Total area included in the SICE and Non-SICE categories are given in square kilometers next to the section heading. Families are ordered by total family catch (given in tonnes). Family catch standardized by square kilometer is also given for comparison. Finally percent of total historical catch is presented for each family for both the SICE and non-SICE seamount groups. Table S6.
Maximum annual family catch from the Watson et al. 2018 database grouped by SICE and Non-SICE seamounts from the Allain seamount subset excluding seamounts with one or more emergent cells in the seamount area (N=177). Families making up more than 1% of maximum annual catch (the catch at a given seamount for it's most productive fishing year) at each SICE and Non-SICE seamounts. Total area included in the SICE and Non-SICE categories are given in square kilometers next to the section heading. Families are ordered by total family catch (given in tonnes). Family catch standardized by square kilometer is also given for comparison. Finally, percent of total historical catch is presented for each family for both the SICE and non-SICE seamount groups.

Movie S1. Monthly Average Chl
Monthly average (over 10 year study period) chlorophyll concentrations around Hotspur Seamount (summit depth 30m, latitude 18°S). White pixels contained depths £ 30m and were removed to prevent incorporation of optically shallow waters into analysis.

Movie S2. Yearly Average Chl
Yearly average (over 10 year study period) chlorophyll concentrations around Hotspur Seamount (summit depth 30m, latitude 18°S). White pixels contained depths £ 30m and were removed to prevent incorporation of optically shallow waters into analysis.