Particulate Organic Matter Supply Research Articles

Benthic O2 uptake by coral gardens at the Condor seamount (Azores)

Using the non-invasive aquatic eddy covariance technique, we provide the first oxygen (O2) uptake rates from within coral gardens at the Condor seamount (Azores). To explore some of the key drivers of the benthic O2 demand, we obtained benthic images, quantified local hydrodynamics, and estimated phototrophic biomass and deposition dynamics with a long-term moored sediment trap. The coral gardens were dominated by the octocorals Viminella flagellum and Dentomuricea aff. meteor. Daily rates of O2 uptake within 3 targeted coral garden sites (203 to 206 m depth) ranged from 10.0 ± 0.88 to 18.8 ± 2.0 mmol m-2 d-1 (mean ± SE) and were up to 10 times higher than 2 local sandy reference sites within the seamount summit area. The overall mean O2 uptake rate for the garden (13.4 mmol m-2 d-1) was twice the global mean for sedimentary habitats at comparable depths. Combined with parallel ex situ incubations, the results suggest that the octocorals might contribute just ~5% of the observed O2 uptake rates. Deposition of particulate organic matter (POM) assessed by the sediment trap accounted for less than 10% of the O2 demand of the coral garden, implying a substantial POM supply circumventing the deployed traps. Our results expand the database for carbon turnover rates in cold-water coral habitats by including the first estimates from these largely understudied coral gardens.

Environmental Considerations for the Management of the Bivalve Fisheries of Bahía Magdalena (Mexico), a Coastal Lagoon at the Southern End of the California Current

This study presents an overview of bivalve assemblages in Bahia Magdalena (BM, México) and the possible impact of environmental variability on these populations, constantly stressed by fishing. This lagoon is responsible for a high proportion of harvest of regional bivalves. First, we list the bivalve species reported in public biogeographic databases. Based on eight commercially exploited species, we described the composition of the bivalve assemblage and its biological characteristics, the history of fishery, and environmental variability in the marine area adjacent to the lagoon (1970–2019) and the habitat of bivalves (2002–2020). Sources of data were public databases and published literature. The enlisted species (n = 184) belong to six orders, and most are small and infaunal, but the structure of the assemblage is unknown. The fisheries began at different times and focused on the most valuable resources. Almost all harvest of bivalves had wide variations because of intensive fishing and a weak regulation frame. After 2015, the main resources were the Pacific wing-oyster (a new resource since 2017) and the geoduck clam due to the declining abundance of other resources (e.g., pen shells, Pacific calico scallop). There was a warming trend in the region since the 1970's, but the strongest El Niño-Southern Oscillation (ENSO) phases caused the most notable changes before 2013; after that year, a combination of large-scale phenomena increased the temperature significantly. The trend of chlorophyll-a abundance negatively correlated with temperature, but there was an almost constant supply of particulate organic matter in the interior of Bahia Magdalena (BM). After 2015, the quality of lagoon water gradually deteriorated, and in 2017 and 2019, harmful algal blooms developed, but the impact was not fully assessed. The challenges faced by the fishery are multiple (institutional weakness and regional warming); however, permanent monitoring programs of environmental conditions and critical biological variables should be implemented to design scenarios that allow fishery sustainability.

Particulate nutrient fluxes over a fringing coral reef: Source‐sink dynamics inferred from carbon to nitrogen ratios and stable isotopes

We examined spatial and temporal variations in particulate organic matter (POM) dynamics over a fringing coral reef (Ningaloo Reef) in Western Australia during the austral autumn and spring. Total POM concentrations generally did not differ between seasons or reef zones, but the composition of POM, in terms of carbon isotope ratios (δ13C‐POM), carbon to nitrogen ratios (C : N), and fatty acids, changed consistently in water flowing across the reef. Both δ13C‐POM and C : N increased from the fore reef to the reef flat and lagoon, −23.0‰ to −20.1‰ and 7.31 to 8.34, respectively. Average rates of net POM uptake by the reef community were highest over the reef crest (4 to 30 mmol N m−2 d−1 and 6 to 130 mmol C m−2 d−1), with a Bayesian isotope model confirming independent measurements of high uptake rates of allochthonous POM (oceanic phyto‐ and zooplankton). In contrast, over the reef flat, net release of POM was observed (−4 to −5 mmol N m−2 d−1 and −50 mmol C m−2 d−1), with gross release rates (estimated as −6 to −8 mmol N m−2 d−1 and −30 to −90 mmol C m−2 d−1) indicating that the release of autochthonous POM may be of similar magnitude to allochthonous uptake. Examining POM dynamics in terms of gross fluxes reinforces the dependence of coral reef systems on oceanographic processes for allochthonous POM supply, as well as highlighting the potential for autochthonous POM production to supply nutrients to benthic and pelagic communities downstream.

Oceanographic forcing of nutrient uptake and release over a fringing coral reef

Nitrate and nitrite (NOx) and phosphate (PO4) dynamics over Ningaloo Reef, Western Australia, are shown to depend on oceanographic forcing of coupled mass transfer limited (MTL) gross uptake and gross release from remineralized oceanic particulate organic matter (POM). Estimates of gross release rates increased significantly with increasing POM uptake and were of the same order as gross uptake rates. Gross uptake rates increased significantly with increasing oceanic concentrations and wave energy dissipation, were 35–80% higher over the reef crest (7–9 mmol NOx m−2 d−1 and 4–5 mmol PO4 m−2 d−1), and were significantly correlated with independent estimates of POM‐mediated gross NOx uptake, supporting both MTL uptake and the strong role of oceanic POM supply. The relative supply of NOx and POM was linked to the seasonal dynamics of a regional current system. In late spring, upwelling associated with seasonally strong equator‐ward winds led to increased NOx concentrations (0.71 ± 0.2 µmol L−1), POM < NOx and the reef was a net nutrient sink (5390 mmol NOx m−1 d−1 and 270 mmol PO4 m−1 d−1). In contrast, during the autumn, NOx was low (0.16 ± 0.06 µmol L−1), but POM > NOx and the reef was a net nutrient source (−7060 mmol NOx m−1 d−1 and −730 mmol PO4 m−1 d−1). The autumn enhancement of oceanic POM supply to the reef can be attributed to a regional phytoplankton bloom associated with acceleration of the oligotrophic Leeuwin Current, which may result in a significant supply of dissolved nutrients to downstream communities.

Detection of low bottom water oxygen concentrations in the North Sea; implications for monitoring and assessment of ecosystem health

Abstract. This paper presents new results from high temporal resolution observations over two years (2007 and 2008) from instrumented moorings deployed in the central North Sea, at the Oyster Grounds and on the northern slope of Dogger Bank (North Dogger). The water column was stratified in the summer at both sites, leading to limited exchange of the water in the bottom mixed layer. Data from these moorings revealed the variable nature of summer oxygen depletion at the Oyster Grounds. The combination of in situ and ship-based measurements allowed the physical and biological conditions leading to decreasing dissolved oxygen concentrations in bottom water to be examined. In 2007 and 2008, the concentration of dissolved oxygen in the bottom water at both sites was observed to decrease throughout the summer period after the onset of stratification. Depleted dissolved oxygen concentration (6.5 mg l−1, 71% saturation) was measured at the North Dogger, a site which is not significantly influenced by anthropogenic nutrient inputs. Lower oxygen saturation (5.2 mg l−1, 60% saturation) was measured for short durations at the Oyster Grounds. The seasonal increase in bottom water temperature accounted for 55% of the decrease in dissolved oxygen concentration at the Oyster Grounds compared to 10% at North Dogger. Dissolved oxygen concentration in bottom water at the Oyster Grounds was shown to be strongly influenced by short term events including storms and pulses of particulate organic matter input. In contrast, dissolved oxygen concentration in bottom water at the North Dogger reflected longer seasonal processes such as a gradual temperature increase over the summer and a more steady supply of particulate organic matter to the bottom mixed layer. The differences between the study sites shows the need for an improved understanding of the mechanisms driving these processes if the use of oxygen in marine management and ensuring ecosystem health is to be meaningful and successful in the future. These high frequency observations provide greater understanding of the nature of the depletion in bottom oxygen concentration in the North Sea.

Extracellular enzyme activity in the Arctic Northeast Water polynya

We studied extracellular enzyme activity (EEA) in samples of the water column and sediments of the Northeast Water polynya (77 to 81 N, 6 to 17' W) during July-August 1992 in order to determine controls on bacterial hydrolytic activity and possible roles of EEA in the fates of particulate organic matter (POM) on an arctic continental shelf. Samples were taken by Niskin bottle, floating sediment trap and box corer from the USCGC 'Polar Sea' at stations throughout the polynya and icecovered surroundings. Fluorogenic substrate analogs were used to estimate potential maximal rates of peptidase, chitobiase and lipase EEA in time course experiments over incubation periods of 1.3 to 14 h at -1, 2 , 5 , 10, and 16OC. Increasing incubation temperature typically increased EEA, but psychrophilic rates (little to no increase upon warming) were detected in several samples, usually for peptidase activity. Rates adjusted to in situ temperature (EEAIS) and normalized to bacterial abundance were compared to similarly determined rates reported previously for other (including temperate) environments and to each other. In general, EEAls rates in polynya waters were lower than in other pelagic environments, while rates in polynya sediments were comparable to those in other sediments. The whole polynya data set, which allows the first comparison between pelagic and benthic rates within a given environment, revealed that EEA, rates (usually peptidase activity), whether normalized to bacterial abundance or to sample dry weight, were generally higher in pelagic samples, particularly in samples from sediment traps deployed in the subzero Polar Surface Water mass. Although relationships between EEAls and temperature or several other environmental parameters (measured by other researchers on the Polar Sea) could not be identified, chitobiase and lipase EEA, did correlate positively, and peptidase activity negatively, with indicators of POM supply to the benthos (pigment concentrations). Observed differences in the effects of incubation temperature and environmental POM supply between enzymes may be explained by differences in location and regulation of peptidase and other EEA.

Organic carbon mineralization in the Santa Catalina Basin: benthic boundary layer metabolism

Organic carbon mineralization rates in the benthic boundary layer (BBL) of the Santa Catalina Basin (1300 m depth) were estimated to identify the primary sites and organisms involved in the turnover of carbon and to compare these rates with the supply of particulate organic matter entering the system from above. Concurrent in situ measurements of macrozooplankton, epibenthic megafauna, and sediment community oxygen consumption, and bacterioplankton and total microbial (microplankton) metabolism were made on 12 dives with DSRV Alvin in November 1984. Pore water and solid phase chemistries, and sediment microbial activity were measured on samples from box cores. Macrozooplankton oxygen consumption, integrated over the 100 m BBL, was 25.8 μmol O 2 m −2 d −1 (0.3 mg C m −2 d −1). Microplankton carbon mineralization rates were 13–29 mg C m −2 d −1 for the BBL with an assimilation efficiency of 80–90%. The estimated oxygen consumption of the dominant population of epibenthic megafauna, Ophiophthalmus normani, at observed densities was 237.8 μmol O 2 m −2 d −1 (2.4 mg C m −2 d −1). Sediment community oxygen consumption was 2776.8 μmol O 2 m −2 d −1 (28.6 mg C m −2 d −1) which is similar to the estimated microbial carbon mineralization estimate for the sediments of 32 mg C m −2 d −1 assuming a 90% assimilation efficiency. These rates were corroborated further by the observed total inorganic carbon pore water gradients from which a mineralization rate of 10–31 mg C m −2 d −1 was estimated. The combined carbon mineralization by the three consumer groups ranged from 25.7 to 63.7 mg C m −2 d −1. For comparison, the oxygen consumption in the BBL based on hydrographic data from the center and sills of the basin was 3.1–20.0 mg C m −2 d −1. Given the large uncertainties in all of the calculations, there was reasonable agreement between these diverse methods of estimating carbon mineralization. The concurrently measured flux of small particulate organic carbon into the BBL using sediment traps was 11.1 ± 2.4 mg C m −2 d −1, providing 17–43% of the estimated carbon mineralized by the three consumer groups.

DECOMPOSITION AND REGENERATION OF NITROGENOUS ORGANIC MATTER IN SEA WATER

1. With a recurrent supply of particulate organic matter, the formation of ammonia, nitrite and nitrate may take place simultaneously. The process which predominates will depend upon the stage at which the new organic matter is introduced. 2. The nature of the suspended particulate matter is of considerable importance in determining the total duration of the decomposition cycle, but the level of its original concentration is only a minor determining factor. 3. There is some evidence of a "growth-promoting" factor, normally effective in the decomposition cycle, but which can be destroyed by high temperature.