Seasonal phosphorus and carbon dynamics in a temperate shelf sea (Celtic Sea)

Abstract

Highlights: Seasonal phosphorus uptake and dissolved organic release examined in the Central Celtic Sea  Uptake highest in spring bloom, with biomass-normalised uptake equal in spring and summer  Release high in November and late spring, with efficient P-retention in summer  Strong phytoplankton influence on spring P-uptake, whilst bacteria influential in summer  Relatively C-rich uptake in November and late April, strongly P-rich in summer AbstractThe seasonal cycle of resource availability in shelf seas has a strong selective pressure on phytoplankton diversity and the biogeochemical cycling of key elements, such as carbon (C) and phosphorus (P). Shifts in carbon consumption relative to P availability, via changes in cellular stoichiometry for example, can lead to an apparent 'excess' of carbon production. We made measurements of inorganic P (P i ) uptake, in parallel to C-fixation, by plankton communities in the Central Celtic Sea (NW European Shelf) in spring (April 2015), summer (July 2015) and fall (November 2014). Short-term (<6 h) P i -uptake coupled with dissolved organic phosphorus (DOP) release, in parallel to net (24 h) primary production (NPP), were all measured across an irradiance gradient designed to typify vertically and seasonally varying light conditions. Rates of P i -uptake were highest during spring and lowest in light-limited fall conditions, although biomass-normalised P i -uptake was similar in spring and summer. The release of DOP was highest in November and declined to low levels in July, indicative of efficient utilization and recycling of the low levels of P i available. Examination of turnover times of the different particulate pools, including phytoplankton and bacteria, indicated a differing seasonal influence of autotrophs and heterotrophs in P-dynamics, with summer conditions associated with a strong bacterial influence and early spring with fast growing phytoplankton. These seasonal changes in plankton composition, coupled with changes in resource availability (P i , light) resulted in seasonal changes in the stoichiometry of NPP to P i -uptake (C:P ratio); from relatively C-rich uptake in November and late April, to P-rich uptake in early April and July. Overall these results highlight how the entire plankton community, both autotrophs and heterotrophs, influence the relative uptake of C and P and that any excess C-consumption relative to the P-rich uptake must be balanced by C-rich process such as the heterotrophic remineralisation and/or consumption of organic material.

Dissolution Dominates Silica Cycling in a Shelf Sea Autumn Bloom

“…During the cruise, water samples were collected from six light depths (60%, 40%, 20%, 10%, 5%, and 1% of surface irradiance) in 20‐L Niskin bottles (OTE: Ocean Test Equipment) on a CTD rosette sampler deployed predawn (0200–0600 local time). Depths were determined by back calculation of the vertical attenuation coefficient ( K d , m −1 ) of photosynthetically active radiation (PAR), with the base of the SML at or close to the depth of the euphotic zone (1% surface irradiance) (Poulton et al, ).…”

“…Depth of the SML was determined from density profiles, from the ships CTD and mooring time‐series, by identifying where potential density increased by 0.02 kg m −3 above the 10‐m value (Poulton et al, ; Wihsgott et al, ). A 2π PAR irradiance sensor (Skye Instruments, SKE 510) on the RRS Discovery and a quantum PAR meter (LiCor Inc., USA) on the mooring‐measured incident irradiance ( E 0 ).…”

“…A 2π PAR irradiance sensor (Skye Instruments, SKE 510) on the RRS Discovery and a quantum PAR meter (LiCor Inc., USA) on the mooring‐measured incident irradiance ( E 0 ). Average SML irradiance ( Ē SML ) was determined following Poulton et al (). Mooring fluorescence data were determined by a Seapoint fluorometer, standardized using fluorescent sulfate microspheres (FluoSpheres, ThermoFisher Scientific) with nighttime data only to avoid daytime quenching (Wihsgott et al, , ).…”

“…Incubations under different daily light doses followed Poulton et al () in a modified refrigeration container, with temperatures ±1–2 °C of those in situ. Irradiance was provided by one to three daylight simulated LED panels (Powerpax, UK), combined with neutral density filters (Lee Filters TM , UK), to achieve target irradiance for each light level (sampling depth).…”

Wind‐Driven Strain Extends Seasonal Stratification

“…The canonical view is that seasonal stratification in the Celtic Sea is a response to a competition between surface heating and vertical mixing processes (tides, wind, and surface heat loss driving convective mixing). In spring a positive net heat flux leads to the onset of stratification (Pingree et al, 1976 and1977;Sharples et al, 2006;Wihsgott et al, 2019) and triggers the spring bloom (Carr et al, 2019;Garcia-Martin et al, 2019;Pingree et al, 1977), which continues until nutrients are depleted in the euphotic layer (Pemberton et al, 2004;Poulton et al, 2019). Throughout summer heat input enhances stratification and later in autumn stratification weakens as heat loss and wind stress deepen the surface mixed layer until reaching a fully mixed water column in winter (Wihsgott et al, 2019).…”