Reduced water motion enhances organic carbon stocks in temperate eelgrass meadows

Abstract

Organic carbon (OC) storage in coastal vegetated ecosystems is increasingly being considered in carbon financing and climate change mitigation strategies. However, spatial heterogeneity in these “blue carbon” stocks among and within habitats has only recently been examined, despite its considerable implications. Seagrass meadows have potential to store significant amounts of carbon in their sediments, yet studies comparing sediment OC content at regional and meadow scales remain sparse. Here, we collected sediment cores from six temperate eelgrass (Zostera marina) meadows on the coast of British Columbia, Canada, to quantify sediment OC stocks, accumulation rates, and sources, and to examine local and regional drivers of variability. Sediment OC content was highly variable—across all sites, stocks in the top 0–5 cm ranged from 83 to 1089 g OC m−2, while the 15–20 cm stocks exhibited a 24‐fold difference, from 59 to 1407 g OC m−2. Carbon accumulation rates ranged from 4 to 33 g OC m−2 yr−1. Isotopic mixing models revealed that sediment OC was primarily terrestrial carbon (41.3%) and canopy‐forming kelps (33.3%), with a smaller contribution of eelgrass (25.3%). Here, we show that regional variability in OC content exceeds meadow‐scale variability. This result is likely driven by landscape factors, most notably relative water motion, representing a more dominant control on seagrass OC accumulation than meadow‐scale factors such as canopy complexity. These findings elicit caution when scaling up seagrass meadow OC content and demonstrate that measures of the hydrodynamic environment could improve estimates of carbon storage in temperate soft sediment habitats.

A Synthesis of Blue Carbon Stocks, Sources, and Accumulation Rates in Eelgrass (Zostera marina) Meadows in the Northeast Pacific

“…Moreover, it is worth noting that in all three locations, the variance in vegetated and unvegetated sediment carbon content was sufficiently large that the observed trends were not definitive (Figure ). The vegetated and unvegetated values observed here are similar to those observed by Postlethwaite et al () and Prentice et al () in Clayoquot Sound, Vancouver Island, and the BC Central Coast, respectively. The observed similarity between vegetated and unvegetated OC content in the PNW may be due to the characteristically shallow root systems of Z. marina , its patchy and sparse nature, and/or its tendency to occupy coarse sandy sediments that are relatively ineffective at retaining carbon (Howard, ), resulting in limited enhancement of OC accumulation relative to bare sediments.…”

“…Our isotopic data suggest that the organic carbon that ends up in eelgrass meadow sediments is largely derived from noneelgrass sources, which aligns with previous findings that upward of 50% of carbon in seagrass sediments may be allochthonous in origin (Kennedy et al, ; Miyajima et al, ; Oreska et al, ; Prentice et al, ). While we did not have enough replicates of sediment isotope values or enough localized signatures of potential carbon sources to determine the relative proportion of carbon sources using an isotope mixing model, the biplot shows a clear separation of signatures in the sediments from those of Z. marina across various regions (Figure ).…”

“…Z. marina also has physical characteristics that may limit its ability to trap and retain sediments. For example, although the canopy can exceed 3 m in our study areas (Bulthuis, ), the leaves are thin and supple, and high currents can lay them quite flat near the sediment surface, nearly eliminating their particle‐trapping ability (Prentice et al, ). In addition, Z. marina rhizomes spread just under the sediment surface, primarily within the top 4 cm (Phillips 1984), which may enhance sediment stability relative to unvegetated areas, but provide much less stability than other seagrass species with thick root mats such as Posidonia oceanica .…”

“… Note . Sources are as follows: 1 = Prentice et al (), (2) Spooner (), (3) Poppe and Rybczyk (), (4) Lutz (2018), and (5) Short et al ().…”

“…We analyzed all available sediment carbon data from eelgrass meadows in the Pacific Northwest, spanning from Southeast Alaska to Southern Oregon. Collections were made independently by a number of research groups and included published manuscripts, theses, and unpublished values (Lutz, 2018; Murray, ; Poppe & Rybczyk, ; Prentice et al, ; Short et al, ; Spooner, ; Stephens & Eckert, ). While the studies varied slightly in their objectives and scope, all studies had a common goal of quantifying carbon stocks, sequestration rates, and sources of stored carbon in Z. marina meadows of the Pacific Northwest, to continue to fill the data gap for this region and species.…”