The autonomous clean environmental (ACE) sampler: A trace‐metal clean seawater sampler suitable for open‐ocean time‐series applications

Abstract

The fundamental role of the micronutrient Fe in controlling phytoplankton growth in large parts of Earth's oceans and a lack of information on seasonal transitions in remote regions motivated us to create an autonomous water sampler capable of collecting uncontaminated open‐ocean seawater samples with monthly resolution over a full annual cycle. Phytoplankton are at the base of the food chain, which take up carbon dioxide through photosynthesis. Assessing Fe availability is essential to understanding both ocean productivity and our climate. This need is particularly important in the Southern Ocean, where Fe limitation is wide spread and access is difficult especially in winter. To address this need, we have developed an autonomous system capable of observing iron concentrations over a full seasonal cycle, at the subnanomolar concentrations that are important in the open ocean. The automated clean environmental sampler has been developed initially for 1‐yr deployments on oceanographic moorings. Twelve samples per unit can be programmed to collect 65 mL of seawater through a noncontaminating, primarily Teflon sample path. The system is vaned to maintain its intake tubes in upstream water and has been tested to 100 m depth. The system was deployed during a GEOTRACES section (SR3 line, GS01) in 2018 alongside an industry standard, trace metal clean rosette, and no significant difference was found in Fe concentrations in the 50 pmol L–1 range, proving its capability at collecting uncontaminated seawater samples in the open ocean at temperatures above 0°C.

Analysis of influencing factors on displacement sampling performance of deep-sea gas-tight sampler

“…From a marine biology and chemistry perspective, trace elements in seawater are critical to our understanding of marine life and the environment. Merwe et al (2019) from the University of Tasmania have developed a water sampler that can observe the concentration of Fe in seawater throughout the seasonal cycle. However, this kind of water collector has only been tested in seawater with a depth of 100 meters, and its operating ability in the deep sea cannot be guaranteed.…”

Southern Ocean phytoplankton dynamics and carbon export: insights from a seasonal cycle approach

“…Deep winter mixing and seasonal iron limitation were revealed as an important driver of the seasonal cycle of ecological succession between phytoplankton-dominated communities in summer and non-algal particles such as detritus that play a significant role in winter [79]. Of note is an autonomous system recently developed for the SOTS mooring that will observe the full seasonal cycle of iron concentrations at subnano molar concentrations shedding light on seasonal variability of iron limitation [80]. These results will complement NPQ estimates of iron stress from fluorescence which have similarly been shown by Schallenberg et al [81] to be indicative of phytoplankton physiological state with respect to iron limitation.…”

Trace Metals

“…In contrast to MITESS, the ACE sampler collects filtered (0.2 µm, polyethersulfone filter membrane) samples into 65 mL fluoropolymer containers. The time-controlled sampler works by drawing seawater through up to 12 individual intake tubes via acidwashed filters into the UHP filled sample bottles using individually programmable micro-peristaltic pumps (density displacement mechanisms; van der Merwe et al 2019). The intake tubes are maintained in an upstream position relative to the device to minimize contamination during sampling (van der Merwe et al 2019).…”

“…The time-controlled sampler works by drawing seawater through up to 12 individual intake tubes via acidwashed filters into the UHP filled sample bottles using individually programmable micro-peristaltic pumps (density displacement mechanisms; van der Merwe et al 2019). The intake tubes are maintained in an upstream position relative to the device to minimize contamination during sampling (van der Merwe et al 2019). A key advantage of this system is that samples are filtered to remove particles, and intake pots are made of PFA which together with their small surface area reduces biofouling and thus potential sampling artefacts and contamination (van der Merwe et al 2019).…”

“…The intake tubes are maintained in an upstream position relative to the device to minimize contamination during sampling (van der Merwe et al 2019). A key advantage of this system is that samples are filtered to remove particles, and intake pots are made of PFA which together with their small surface area reduces biofouling and thus potential sampling artefacts and contamination (van der Merwe et al 2019). However, a disadvantage of the ACE system is the fact that the filtered samples can only be acidified back in the laboratory after recovery of the sampler which might result in low-biased results due to wall absorption.…”