Size fractionated biogeochemical constituents across adjacent coastal systems informs approaches for integrating small catchment studies into regional models

Abstract

Observed and predicted hydrological changes in C‐rich boreal ecosystems have the ability to change the transport trajectory of biogeochemical constituents from the land to ecologically, economically, and culturally important coastal systems. Yet, most of our current understanding of biogeochemical fluxes and cycling across salinity gradients stem from observations of large and urbanized riverine systems, which overlooks the numerically abundant smaller systems. In this study, we conducted a baseline assessment of the biogeochemical constituents across salinity gradients among two adjacent small systems in the boreal zone. Dissolved iron (DFe) and its ratio with dissolved organic carbon (DFe : DOC) were the most sensitive indicators for small catchment heterogeneity. These parameters were the best indicators of change among coastal systems across regional and seasonal scales. Our results also confirm consistencies in common optical measures (SUVA254 and S[275–295/350–400]) and DOC to nitrogen ratios that may adequately provide representation of biogeochemical composition on a regional scale. Simultaneous variation in biogeochemical parameters across particulate and dissolved pools during the summer‐to‐fall transition period indicate this as an important timeframe for targeted investigation of the linkages between biogeochemical parameters and coastal ecosystem functioning. By providing some key spatial and temporal constraints on biogeochemical fluxes among boreal river‐estuaries, our findings indicate that DFe and DFe : DOC ratios should be used to design research aimed at capturing regional and coastal ecosystem scale biogeochemical fluxes to inform Earth System Models.

The Role of Boreal Seagrass Meadows in the Coastal Filter

“…Comparing δ 13 C org against source data from Khoo et al ( 2023) suggests surface sediment in and around each eelgrass meadow was a combination of terrestrial and marine derived sources, but predominantly marine derived (Figure 6a). This was supported by the C:N ratio, which also overlapped with the marine and terrestrial source C:N ratios reported in Khoo et al (2023), and in some cases even overlapped with the eelgrass C:N ratio (i.e., Swift Current) and the Redfield ratio for plankton (C:N of 6.6; Redfield, 1958) (Figure 6b). However, C org was predominantly supplied from marine sources other than the eelgrass themselves, given that δ 13 C org in the seagrass tissue (−11.64 ± 1.8‰) was overall ∼10‰ more enriched than the surface sediment (Figure 6a).…”

“…Swift Current is located in the most developed region of the three sites, where there is a community <1 km from the meadow (population of 207; Statistics Canada, 2022), but on the opposite side of the inlet. Freshwater entering the system is carbon-rich, as it primarily originates from boreal forest draining catchments (Khoo et al, 2023). Eelgrass at the three sites is exposed predominantly to semidiurnal tides and estuarine circulation with lower salinity at low tide and higher salinities at high tide.…”

“…We could not run a mixture model analysis for our stable isotope data because we only had source values for eelgrass tissue δ 13 C org and δ 15 N. Instead, isotope values were compared to the eelgrass tissue isotopic signatures and δ 13 C org values from Khoo et al (2023), where isotope values of suspended particles were measured from two freshwater samples and two saltwater samples collected from both Piper's Hole and Come by Chance in Placentia Bay (Figure 1a). Given that the three meadows are surrounded by forest and anthropogenic infrastructure (e.g., cottage or road), we would expect terrestrial sources include tree detritus, anthropogenic pollution, or bacteria.…”