Vertically Resolved Pelagic Particle Biomass and Size Structure Across a Continental Shelf Under the Influence of a Western Boundary Current. Issue 2 (18th February 2023) - Schilling, Hayden T.
Abstract: Continental shelves are key to societal interactions with the oceans, supporting >90% of the world's fisheries through highly productive ecosystems. Previous research has shown that phytoplankton biomass is generally higher on the inner continental shelves, often due to increased nutrient inputs from upwelling or coastal run‐off. However, consistency in observed vertical and horizontal gradients (in abundance, biomass or size) of larger particulates, including zooplankton, on continental shelves has not been established. Using an optical plankton counter and CTD mounted on an undulating towed body, we present high‐resolution vertically resolved profiles of pelagic particle size structure across a continental shelf. Biomass was highest inshore, declining with distance from shore and with depth in the top 100 m of the water column, although the presence of frontal zones can alter this pattern. In the region adjacent to the East Australian Current (EAC), uplift generated by either the EAC interacting with the continental slope or upwelling‐favorable winds, correlated with smaller geometric mean sizes and steeper size spectrum slopes, particularly in the presence of frontal features. South of the EAC separation, the continental shelf water mass was more homogenous but still displayed the same horizontal and vertical patterns in particulate biomass and mean size. By combining our observations in a global comparison, we demonstrate consistent particulate distributions on continental shelves where the inner shelf has higher biomass with a steeper size spectrum slope compared to offshore. The highly productive inner shelf supports zooplankton communities vital to temperate ecosystems and coastal fisheries, through their consistently high biomass. Plain Language Summary: It is commonly accepted that chlorophyll a and phytoplankton are more abundant on continental shelves compared to the open ocean due to nutrient enrichment from either upwelling or terrestrial inputs. Little is known about the consistency in larger particulates, including zooplankton. We present the first depth resolved cross‐shelf transects of zooplankton sized particulates. We show globally consistent patterns where close to the coast there is higher abundance and biomass, smaller geometric mean size and a steeper size spectrum slope representing a more productive community. Key Points: We present the first depth resolved cross‐shelf transects of the abundance, biomass, and geometric mean size of particulate matter Higher biomass and abundance were generally found close to the coast likely representing a more productive community These patterns were consistent globally but can show significant small‐scale variation around frontal zones