The present study was carried out in six fjords within New Zealand’s Fiordland system, specifically Breaksea Sound, Chalky Inlet, Doubtful Sound, Dusky Sound, Long Sound, and Wet Jacket Arm, as described in Tobias-Hünefeldt et al., 2019. Analyses were divided into three categories: 1) a multi-fjord analysis comprising five of the tested fjords (excluding Long Sound), 2) a high resolution study along Long Sound’s horizontal axis, and 3) a depth profile of Long Sound’s deepest location. Total community composition (via 16S and 18S gene sequencing) and metabolic potential did not significantly covary across the five studied fjords (Mantel, r < 0.01, P = 0.47), Long Sound’s horizontal transect (Mantel, r < 0.01, P > 0.05), or Long Sound’s depth profile (Mantel, r < 0.22, P > 0.05). However, depth covaried with community structure for five studied fjords, across the horizontal transect at Long Sound, and along Long Sound’s depth profile (Fig. 1, Figure S1-S3, Table S1). Microbial community changes observed along the horizontal axis were stronger between surface and 10 m communities (Mantel, Multifjord – r = 0.21, P < 0.01, Transect – prokaryotes r = 0.47, P = < 0.01, eukaryotes r = 0.56, P < 0.01), as opposed to along the horizontal axis of individual fjords (Mantel, Multifjord – r = 0.08, P = 0.04, Transect – prokaryotes r = 0.21, P = 0.01, eukaryotes r = 0.13, P = 0.07) (Figure S2-3). Additionally, significant differences in metabolic potential in response to depth were observed both across multiple fjords (Anosim: R = 0.10, P = 0.03) and along a transect from the entrance of the ocean to the head of Long Sound (Anosim: R = 0.27, P = < 0.01) (Fig. 1).
Across the five fjords (excluding Long Sound), surface samples were more metabolically active (i.e., average metabolic rate [AMR]) compared to the 10 m samples (Wilcox test, W = 425, P < 0.01). Samples closer to the fjord head displayed increased metabolic rates (Wilcox test, W = 0, P < 0.01). Thus, metabolic variability varied with horizontal sampling location (Fig. 1b). While activity was not consistent along the length of Long Sound, surface samples in the low salinity layer were more metabolically active than those collected at 10 m (Figure S4). Heterotrophic production (via leucine incorporation) was not significantly correlated with microbial abundance within the five studied fjords and Long Sounds horizontal axis (Mantel – Multifjord r = 0.04, P = 0.22, Horizontal r = 0.04, P = 0.32). Along the depth profile, prokaryotic abundance and production were significantly correlated (Mantel, r = 0.60, P = 0.01) exhibiting a large drop in productivity from the surface to 10 m followed by a more gradual decrease.
To further explore the depth-associated changes we studied a high resolution depth profile of the deepest fjord (Fig. 2). We hypothesized that metabolic rate and diversity would be driven by marine snow linked to photosynthetic primary producers at the surface (e.g. phytoplankton and macroalgae; Fig. 2a) leading to a steady decrease in metabolic potential as resources were depleted with increases in depth. Any deviation altering the slow loss of metabolic potential would be linked to extraneous sources of nutrients uncoupled from surface activity (i.e. benthic influences, subsidies from land-based inputs). We observed a steady loss of metabolic diversity, and rate, from the surface to 100 meters (Fig. 2b, c), with sustained increases at depths past 100m. However, abundance did not follow the same pattern, and instead continuously decreased until 360 m (Fig. 2d). Abundance and metabolic changes over depth were associated with shifts in specific carbon utilization potential, where carbohydrate metabolism decreased from 12.7–6.8%, as carboxylic acid utilization increased from 12.0–29.5% from the surface to 360 m (Fig. 2e). This likely reflected the diminishing abundance of readily mineralizable substrates with depth, and the increase in recalcitrant sources of carbon and energy. Consistently, we also observed increases in phosphorylated chemical metabolism peaking at 40 and 360 m (Fig. 2e) as expected from utilization of phosphorous at the surface during blooms 23. However, observed changes in metabolic potential did not reflect changes in prokaryotic or eukaryotic community composition, suggesting that while the community members were relatively consistent past a certain depth (i.e., 10 m for eukaryotes and 40 m for prokaryotes) the metabolic potential changed dynamically past 100 m, regaining peak metabolic potential with proximity to the bottom (Fig. 2f).
Our results demonstrate that metabolic potential and activity in fjords is linked to similar parameters as microbial community composition across surface or near surface sites. However, distinct selective pressures exist at aphotic sites which ultimately affect the link between phylogenetic and metabolic diversity. The observed patterns are contrary to the open ocean carbon pump paradigm and demonstrate that additional refractory sources of organic matter, including resuspension of terrestrial organic matter associated with benthic communities, are important contributors to microbial activity in fjords, which form a major marine biome worldwide (e.g. Patagonian, Scandinavian, Northeastern Pacific systems). We propose that this reflects the influence of the benthic microbial loop and incorporation and breakdown of terrestrial organic matter in fjordic sediments. Sediment resuspension can occur through a variety of abiotic 24,25 and biotic sources (known as bioturbation, 26). The resuspension of organically rich sediments has previously been shown to increase microbial activity 27. Observed patterns suggest that resuspension could also be driven by bottom feeding organisms, increasing suspended organic matter and its utilization in near bottom habitats 28. Therefore, organic matter sources influence the relationship between microbial communities and their metabolic potential.