M22 D21 Research Articles

Biological‐ and physical‐induced oxygen dynamics in melting sea ice of the Fram Strait

We investigated the production, consumption, and exchange of O2 in melting sea ice to assess the biological‐ and physical‐induced O2 turnover. The underside of the ice was covered with 5–20 cm3 large, buoyant algal aggregates. Their gross primary production amounted to 0.49 mmol C m−2 d−1, which was 4.5 times higher than the primary production of sea ice—encrusted microalgae (0.11 mmol C m−2 d−1). The phototrophic biomass of the aggregates (2.94 mg chlorophyll a m−2) was six times higher than that encountered in the sea ice itself. Taxono‐specific investigations strongly suggest that the aggregates were formed from agglutinated algae released from the melting ice. At the prevailing light conditions, the sea ice—encrusted communities were almost at metabolic balance, while the aggregates were net heterotrophic. Together, the two communities were responsible for an overall O2 consumption of 0.32 mmol m−2 d−1. The sea ice—associated communities thereby represent a southward‐drifting carbon source that is being exhausted by sea ice—affiliated food webs. The sea ice volume decreased rapidly, releasing meltwater at a rate 25 L m−2 d−1, but no surface melt ponds were formed. Aquatic eddy correlation (EC) measurements on the underside of the ice revealed a light‐dependent O2 exchange rate. However, the integrated signal resolved a net O2 uptake of 7.70 mmol m−2 d−1. The net O2 exchange was therefore dominated by the production of O2‐depleted meltwater rather than biological activity. The EC technique represents a new non‐invasive tool for O2 studies in sea ice communities.

Large salp bloom export from the upper ocean and benthic community response in the abyssal northeast Pacific: Day to week resolution

A large bloom of Salpa spp. in the northeastern Pacific during the spring of 2012 resulted in a major deposition of tunics and fecal pellets on the seafloor at ∼ 4000 m depth (Sta. M) over a period of 6 months. Continuous monitoring of this food pulse was recorded using autonomous instruments: sequencing sediment traps, a time‐lapse camera on the seafloor, and a bottom‐transiting vehicle measuring sediment community oxygen consumption (SCOC). These deep‐sea measurements were complemented by sampling of salps in the epipelagic zone by California Cooperative Ocean Fisheries Investigations. The particulate organic carbon (POC) flux increased sharply beginning in early March, reaching a peak of 38 mg C m−2 d−1 in mid‐April at 3400 m depth. Salp detritus started appearing in images of the seafloor taken in March and covered a daily maximum of 98% of the seafloor from late June to early July. Concurrently, the SCOC rose with increased salp deposition, reaching a high of 31 mg C m−2 d−1 in late June. A dominant megafauna species, Peniagone sp. A, increased 7‐fold in density beginning 7 weeks after the peak in salp deposition. Estimated food supply from salp detritus was 97–327% of the SCOC demand integrated over the 6‐month period starting in March 2012. Such large episodic pulses of food sustain abyssal communities over extended periods of time.

Paradox reconsidered: Methane oversaturation in well‐oxygenated lake waters

The widely reported paradox of methane oversaturation in oxygenated water challenges the prevailing paradigm that microbial methanogenesis only occurs under anoxic conditions. Using a combination of field sampling, incubation experiments, and modeling, we show that the recurring mid‐water methane peak in Lake Stechlin, northeast Germany, was not dependent on methane input from the littoral zone or bottom sediment or on the presence of known micro‐anoxic zones. The methane peak repeatedly overlapped with oxygen oversaturation in the seasonal thermocline. Incubation experiments and isotope analysis indicated active methane production, which was likely linked to photosynthesis and/or nitrogen fixation within the oxygenated water, whereas lessening of methane oxidation by light allowed accumulation of methane in the oxygen‐rich upper layer. Estimated methane efflux from the surface water was up to 5 mmol m−2 d−1. Mid‐water methane oversaturation was also observed in nine other lakes that collectively showed a strongly negative gradient of methane concentration within 0–20% dissolved oxygen (DO) in the bottom water, and a positive gradient within ≥ 20% DO in the upper water column. Further investigation into the responsible organisms and biochemical pathways will help improve our understanding of the global methane cycle.

Pelagic metabolism in the waters of the Great Barrier Reef

Pelagic metabolism (community respiration [CR] and net community production [NCP]) were measured in oxygen flux experiments at 14 locations in waters of the Great Barrier Reef (GBR) lagoon between 16°S and 23°S during the dry seasons of 2008 and 2009 and the wet seasons of 2009 and 2010. These were compared to similar experiments conducted in the wet season of 2005 in the far northern GBR (14°S). Seasonality (wet and dry season) was the greatest determinant of respiration rate, with median CR values of 1.85 mmol O2 m−3 d−1 in the dry season and 2.87 mmol O2 m−3 d−1 in the wet season. NCP normally ranged up to 9.16 mmol O2 m−3 d−1, depending on depth and phytoplankton biomass, though an extreme value of 14.96 mmol O2 m−3 d−1 occurred during flood conditions. Cross‐shelf effects had a strong influence on physicochemical variables but weaker effects on both CR and NCP, though both were higher inshore. All but two stations were net autotrophic, with a median ratio of gross primary production : CR of 1.5. Area‐specific CR was 72 ± 23 standard deviation (SD) mmol O2 m−2 d−1 in the dry season and 94 ± 27 SD mmol O2 m−2 d−1 in the wet season. Area‐specific NCP was lower in the dry season (41 ± 33 SD mmol O2 m−2 d−1) than in the wet season (55 ± 54 SD mmol O2 m−2 d−1). Peaks in area‐specific NCP were associated with floods, intrusions of nutrient‐rich sub‐thermocline Coral Sea water, and localized phytoplankton blooms but otherwise showed few seasonal or spatial trends. Overall, CR in GBR lagoon waters was comparable to rates in oligotrophic oceanic waters, but NCP was more typical of shelf systems.

Eddy correlation measurements of oxygen fluxes in permeable sediments exposed to varying current flow and light

Based on noninvasive eddy correlation measurements at a marine and a freshwater site, this study documents the control that current flow and light have on sediment‐water oxygen fluxes in permeable sediments. The marine sediment was exposed to tidal‐driven current and light, and the oxygen flux varied from night to day between −29 and 78 mmol m−2 d−1. A fitting model, assuming a linear increase in oxygen respiration with current flow, and a photosynthesis‐irradiance curve for light‐controlled production reproduced measured fluxes well (R2 = 0.992) and revealed a 4‐fold increase in oxygen uptake when current velocity increased from ∼ 0 to 20 cm s−1. Application of the model to a week‐long measured record of current velocity and light showed that net ecosystem metabolism varied substantially among days, between −27 and 31 mmol m−2 d−1, due to variations in light and current flow. This variation is likely typical of many shallow‐water systems and highlights the need for long‐term flux integrations to determine system metabolism accurately. At the freshwater river site, the sediment‐water oxygen flux ranged from −360 to 137 mmol m−2 d−1. A direct comparison during nighttime with concurrent benthic chamber incubations revealed a 4.1 times larger eddy flux than that obtained with chambers. The current velocity during this comparison was 31 cm s−1, and the large discrepancy was likely caused by poor imitation by the chambers of the natural pore‐water flushing at this high current velocity. These results emphasize the need for more noninvasive oxygen flux measurements in permeable sediments to accurately assess their role in local and global carbon budgets.

Groundwater‐derived dissolved inorganic and organic carbon exports from a mangrove tidal creek: The missing mangrove carbon sink?

A majority of the global net primary production of mangroves is unaccounted for by current carbon budgets. It has been hypothesized that this “missing carbon” is exported as dissolved inorganic carbon (DIC) from subsurface respiration and groundwater (or pore‐water) exchange driven by tidal pumping. We tested this hypothesis by measuring concentrations and δ13C values of DIC, dissolved organic carbon (DOC), and particulate organic carbon (POC), along with radon (222Rn, a natural submarine groundwater discharge tracer), in a tidal creek in Moreton Bay, Australia. Concentrations and δ13C values displayed consistent tidal variations, and mirrored the trend in 222Rn in summer and winter. DIC and DOC were exported from, and POC was imported to, the mangroves during all tidal cycles. The exported DOC had a similar δ13C value in summer and winter (∼ −30‰). The exported δ13C‐DIC showed no difference between summer and winter and had a δ13C value slightly more enriched (∼ −22.5‰) than the exported DOC. The imported POC had differing values in summer (∼ −16‰) and winter (∼ −22‰), reflecting a combination of seagrass and estuarine particulate organic matter (POM) in summer and most likely a dominance of estuarine POM in winter. A coupled 222Rn and carbon model showed that 93–99% of the DIC and 89–92% of the DOC exports were driven by groundwater advection. DIC export averaged 3 g C m−2 d−1 and was an order of magnitude higher than DOC export, and similar to global estimates of the mangrove missing carbon (i.e., ∼ 1.9–2.7 g C m−2 d−1).

Carbon cycling hysteresis in permeable carbonate sands over a diel cycle: Implications for ocean acidification

Dissolved inorganic carbon, dissolved oxygen, H+, and alkalinity fluxes from permeable carbonate sediments at Heron Island (Great Barrier Reef) were measured over one diel cycle using benthic chambers designed to induce advective pore‐water exchange. A complex hysteretic pattern between carbonate precipitation and dissolution in sands and the aragonite saturation state (ΩAr) of the overlying chamber water was observed throughout the incubations. During the day, precipitation followed a hysteretic pattern based on the incidence of photosynthetically active radiation with lower precipitation rates in the morning than in the afternoon. The observed diel hysteresis seems to reflect a complex interaction between photosynthesis and respiration rather than ΩAr of the overlying water as the main driver of carbonate precipitation and dissolution within these permeable sediments. Changes in flux rates over a diel cycle demonstrate the importance of taking into account the short‐term variability of benthic metabolism when calculating net daily flux rates. Based on one diel cycle, the sediments were a net daily source of alkalinity to the water column (5.13 to 8.84 mmol m−2 d−1, depending on advection rates), and advection had a net stimulatory effect on carbonate dissolution. The enhanced alkalinity release associated with benthic metabolism and pore‐water advection may partially buffer shallow coral reef ecosystems against ocean acidification on a local scale.

Silicon uptake and supply during a Southern Ocean iron fertilization experiment (EIFEX) tracked by Si isotopes

We studied the evolution of the isotopic composition of dissolved Si (δ30SiDSi) and biogenic Si (δ30SiBSi) during a diatom bloom induced by an iron fertilization experiment in a Southern Ocean (SO) eddy. Dissolved Si (DSi) consumption was clearly boosted as a result of iron addition. We estimate an apparent Si isotopic fractionation factor of 21.36% ± 0.11%, not significantly different from estimates based on in vitro incubations under Fe‐replete conditions and in agreement with previous SO studies under Fe‐depleted conditions. The temporal variations of the δ30SiSi are best reconstructed considering the fertilized surface waters as an open system, i.e., DSi is continuously supplied from the winter water. Using vertical diffusivity, we estimate a supply of 6.7 ± 1.2 mmol Si m−2 d−1 representing 30% ± 5% of total DSi demand. We estimate biogenic silica (BSi) production at 22.1 ± 1.8 mmol Si m−2 d−1, similar to BSi production under natural conditions in the SO. The DSi use preceding the start of the fertilization was conservatively estimated at 49% ± 4%, and as a result of the Fe‐fertilization, a further 31% ± 4% of the initial DSi reservoir was consumed. The BSi export was estimated to be 17.4 ± 1.7 mmol Si m−2 d−1. Our results suggest that for this study three main processes were acting significantly on δ30SiSi signatures, biological uptake, Si supply from subsurface, and export of BSi, and that the magnitude of production and export in such a Fe‐fertilized experiment are similar to those for natural diatom blooms occurring in the Southern Ocean.

Planktonic and whole system metabolism in a nutrient-rich estuary (the Scheldt estuary)

Planktonic gross primary production (GPP), community respiration (CR), and nitrification (NIT) were measured monthly in the Scheldt estuary by the oxygen incubation method in 2003. No significant evolution of planktonic GPP was observed since the 1990s with high rates in the freshwater area (salinity 0; 97 6 65 mmol C m 22 d 21 ) decreasing seaward (22-37 mmol C m 22 d 21 ). A significant decrease of NIT was observed with regard to previous investigations although this process still represents up to 20% of total organic matter production in the inner estuary. Planktonic CR was highest in the inner estuary and seemed to be mainly controlled by external organic matter inputs. Planktonic net community production was negative most of the time in the estuary with values ranging from 2300 to 165 mmol C m 22 d 21 . Whole estuary net ecosystem production (NEP) was investigated on an annual scale using the results mentioned above and published benthic metabolic rates. A NEP of 239 6 8 mmol C m 22 d 21 was estimated, which confirms the strong heterotrophic status of this highly nutrified estuary. NEP rates were computed from June to December 2003 to compare with results derived from the Land-Ocean Interaction in the Coastal Zone budgeting procedure applied to dissolved inorganic phosphorus and carbon (DIP and DIC). DIP budgets failed to provide realistic estimates in the inner estuary where abiotic processes account for more than 50% of the nonconservative DIP flux. DIC budgets predicted a much lower NEP than in situ incubations (2109 6 31 versus 242 6 9 mmol C m22 d21) although, as each approach is associated with several critical assumptions, the source of

Nutrient retention in a point-source-enriched stream

The capacity of a 3rd-order Ozark Plateau stream (Arkansas, USA) to take up (or remove) nutrient inputs from a rural wastewater treatment plant (WWTP) was examined using nutrient spi- raling methods. Short-term nutrient additions often are used to assess nutrient uptake length, where an exponential decline in the concentration of the added nutrient reflects gross nutrient uptake. We applied this quantitative framework using WWTP effluent as a stream nutrient addition, and esti- mated net nutrient uptake length (Snet), mass transfer coefficient (vf-net), and uptake rate (Unet )i n Co- lumbia Hollow, Arkansas. Water samples were collected at a reference site upstream of the WWTP input and at 6 sites downstream of the WWTP (0.3-2.7 km). Input from the WWTP significantly increased discharge, temperature, conductivity, soluble reactive P (SRP), and NH4-N, and decreased pH and NO3-N 0.3 km downstream from the point source. When P additions from the WWTP were low, stored SRP was released from the stream reach to maintain high water-column concentrations. Dissolved inorganic N was not retained in Columbia Hollow. Most or all of the NH4-N added from the point source was converted to NO3-N, resulting in net nitrification rates of 7 to 31 g NO3-N m22 d21. The relationship between dilution-corrected concentrations and distance from the WWTP input indicated no significant nutrient retention, or that several stream kilometers were required before N and P were taken up. Unet typically was .7- to 10-fold higher and vf-net estimates were 10- to 100-fold lower than values reported for undisturbed streams, indicating low relative nutrient demand. Rather than acting as a nutrient sink, Columbia Hollow appeared to be acting as a short-term storage zone for P and a transformer of N. Thus, the effect of this rural WWTP on the stream was profound, distorting N and P cycling in Columbia Hollow.

Nitrate sources and sinks in Elkhorn Slough, California: Results from long-term continuous in situ nitrate analyzers

Nitrate and water quality parameters (temperature, salinity, dissolved oxygen, turbidity, and depth) were measured continuously with in situ NO3 analyzers and water quality sondes at two sites in Elkhorn Slough in Central California. The Main Channel site near the mouth of Elkhorn Slough was sampled from February to September 2001. Azevedo Pond, a shallow tidal pond bordering agricultural fields further inland, was sampled from December 1999 to July 2001. Nitrate concentrations were recorded hourly while salinity, temperature, depth, oxygen, and turbidity were recorded every 30 min. Nitrate concentrations at the Main Channel site ranged from 5 to 65 mM. The propagation of an internal wave carrying water from ;100 m depth up the Monterey Submarine Canyon and into the lower section of Elkhorn Slough on every rising tide was a major source of nitrate, accounting for 80-90% of the nitrogen load during the dry summer period. Nitrate concentrations in Azevedo Pond ranged from 0-20 mM during the dry summer months. Nitrate in Azevedo Pond increased to over 450 mM during a heavy winter precipitation event, and interannual variability driven by differences in precipitation was observed. At both sites, tidal cycling was the dominant forcing, often changing nitrate concentrations by 5-fold or more within a few hours. Water volume flux estimates were combined with observed nitrate concentrations to obtain nitrate fluxes. Nitrate flux calculations indicated a loss of 4 mmol NO3 m 22 d21 for the entire Elkhorn Slough and 1 mmol NO3 m22 d21 at Azevedo Pond. These results suggested that the waters of Elkhorn Slough were not a major source of nitrate to Monterey Bay but actually a nitrate sink during the dry season. The limited winter data at the Main Channel site suggest that nitrate was exported from Elkhorn Slough during the wet season. Export of ammonium or dissolved organic nitrogen, which we did not monitor, may balance some or all of the NO3 flux.