Resource Availability and Entrainment Are Driven by Offsets Between Nutriclines and Winter Mixed‐Layer Depth

Abstract

While phytoplankton play a key role in ocean biogeochemical cycles, the availability and supply pathways of resources that support their growth remain poorly constrained. Here, we show that the availability of various resources varies over several orders of magnitude throughout the Atlantic Ocean, causing regional contrasts in resource deficiency. Regional variations in the relative availability of nitrogen, phosphorous, silicon, iron, zinc, manganese, cobalt, and cadmium are important and result from the contrasts between winter mixing depths and differences in vertical profiles of the different resources. The winter-time thickening of the mixed layer may replenish or deplete resources via entrainment, depending on the vertical nutrient profile. For nutrients like nitrate, phosphate, and cadmium, entrainment is a consistent source term. While for others, such as manganese and iron, entrainment can reduce ocean resource availability, particularly in subtropical regions. Any future change to the depth of winter-time mixing will cause region-specific changes in relative availability of different resources that may have important ecological consequences.

Circumpolar Deep Water and Shelf Sediments Support Late Summer Microbial Iron Remineralization

“…Consequently, this region may be considered potentially Fe deficient as Fe* calculations for every measurement (all stations and depths) equaled less than 1. While our dFe and PO 4 3 − concentrations are in line with those reported for similar surveys (Rigby et al., 2020), it should be noted that Fe/PO 4 3 − assimilation rates may vary from the globally averaged rate of 0.47 (Sunda & Huntsman,, 1995). Further work is required to identify the nutrient uptake rates of phytoplankton groups specific to this area to determine the accurate extent of Fe limitation.…”

Trace Element Biogeochemistry in the High‐Latitude North Atlantic Ocean: Seasonal Variations and Volcanic Inputs

“…Consequently, the winter mixing was deeper than for example the ferricline or nutricline (Rigby et al, 2020; Tagliabue et al, 2014). This was also the case for the scavenged‐type element Mn, which in many ocean regions shows vertical profiles with a depletion with depth due to scavenging, resulting in a dilution by winter mixing of surface water stocks in the Atlantic between 40°S and 40°N (Rigby et al, 2020). The positive Mn supply through convective mixing was related to the surface ocean depletion observed for DMn in this study (Figure 4) which is then resupplied during winter from deeper water pools.…”

“…The convective supply of dissolved Co, Mn, Ni, Cd, Cu, Pb, Zn, and Al all showed positive values (Table 5), indicating that subsurface stocks with elevated concentrations were accessed during deep winter mixing, which resulted in a replenishment of depleted surface water stocks. Consequently, the winter mixing was deeper than for example the ferricline or nutricline (Rigby et al, 2020; Tagliabue et al, 2014). This was also the case for the scavenged‐type element Mn, which in many ocean regions shows vertical profiles with a depletion with depth due to scavenging, resulting in a dilution by winter mixing of surface water stocks in the Atlantic between 40°S and 40°N (Rigby et al, 2020).…”

“…For our study region, assuming it behaves as the global mean, this would likely result in a reduced supply of nutrients and trace elements, and therefore a reduction in productivity and potentially carbon export. Projections for the North Atlantic subpolar gyre indicated that with a 20% reduction in maximum deep winter mixing depth, a reduced supply of all elements will occur, with the exception of scavenged-type elements, where a reduced dilution of elevated surface ocean stocks might result (Rigby et al, 2020).…”

Upwelled plankton community modulates surface bloom succession and nutrient availability in a natural plankton assemblage

“…This makes prediction of coastal phytoplankton productivity a particular challenge as the entrainment of subsurface populations depends on depth and rates of water mass transport and modulation of source water in either depth or location will likely reflect in altered ecology of bloom populations in coastal waters, and potential biogeochemical changes in nutrient cycling. This would be in addition to the variability in nutrient content of upwelled water sources via ENSO (Espinoza-Morriberón et al, 2017) and variability in the mixed layer depth (Rigby et al, 2020). It is even possible that this uncertainty would be amplified if this response occurred in a more complete food web with larger predators.…”