The effect of biogenic irrigation intensity and solute exchange on diagenetic reaction rates in marine sediments

The activities of infaunal macrobenthos strongly influence the pathways, rates, and extent of organic matter remineralization and associated reactions in marine sediments. Solute transport during irrigation is a particularly important process that stimulates microbial activity and net remineralization, both within and adjacent to the bioturbated zone. Part of the stimulation proximal to the bioturbated zone is due to redox oscillation and oxidant supply during transport, but part of both the near and far-field effects are a result of other factors. Experiments designed to simulate different degrees of diffusive exchange, and thus infaunal abundances or activity, demonstrate a regular and strong dependence of anaerobic remineralization on diffusive transport. For example, net production of NH4+, HPO4=, I-, and Mn++ increases as the effective distance between burrows becomes ≲2 cm (burrow abundance ≳ 800 m-2) in otherwise identical anoxic sediment. Corresponding changes in sedimentary bacterial numbers, exoenzyme activity, per cell growth rate (RNA), and solid phase properties (N, C/N, P) indicate that the increases in net rates are due in part to an absolute increase in total production. Transport-reaction models and experimental results demonstrate that relative decreases in the uptake of solutes into biomass, abiogenic precipitation reactions, and increased removal of inhibiting metabolites all occur simultaneously, enhancing both total and net remineralization. The phenonomenological first-order rate constant for organic matter decomposition is therefore a function not only of the reductant and oxidant pool properties, but also the environmental transport regime. Solid phase reaction products can differ substantially as a function of diffusive openness. For example, both organic P and the organic P/inorganic P ratio increase in more diffusively-open (irrigated) compared to diffusively-closed, anoxic sediment. The sensitivity of solute concentrations, microbial activity and diagenetic reaction balances to diffusive transport regime, indicates that macrofauna can functionally manipulate these properties through relatively small changes in burrow spacing patterns and individual burrow geometries

Oxic and anoxic decomposition of tubes from the burrowing sea anemone Ceriantheopsis americanus: Implications for bulk sediment carbon and nitrogen balance

Many marine infaunal animals form organic tube and burrow linings. The role of these materials in organic matter cycling and preservation in sediments is largely unknown. In the case examined here, the infaunal sea anemone, Ceriantheopsis americanus, (a common component of bottom communities along the east coast of North America) forms a leathery, fibrous tube lining 2–3 mm thick, ∼1 cm in diameter, and typically extending 20–30 cm into deposits. Tube fibers (∼2 mm long, 2–5 μm thick) formed from discharged specialized nematocyst cells, ptychocysts, are composed of a silk-like protein copolymer, cerianthin. Tubes incubated under oxic and anoxic conditions over a period of 122 days demonstrate that initial rates of whole tube decay are 10–100 times slower than usually found for fresh planktonic debris and aquatic macrophytes despite a relatively low molar C:N ratio of ∼5.1. First order decomposition rate constants in oxic water, anoxic water and anoxic sediment are ∼0.76, ∼0.41 and ∼0.22 yr–1 for particulate tube carbon and ∼0.2, ∼0.1 and ∼0.1 yr–1 for particulate nitrogen, respectively (20°C). There are no obvious (under SEM) morphological changes in tube fibers during initial tube decomposition, implying slower long term rates. Although slow, tube decomposition stimulates bacterial activity in sediments from below ∼10 cm depth where any organic matter present is even more refractory than the tubes themselves. In central Long Island Sound muds, tubes apparenlly account for a minimum of ∼0.6–1.8% and 2.8–8.4% of the steady state C and N detrital pools in the upper 10–30 cm of the sediment. C. americanus tube production apparently accounts for ∼9% of the average particulate carbon and ∼12% of the nitrogen fluxes to the benthos. Tube construction by infaunal benthos may thus represent an important pathway for refractory compound formation and organic matter preservation

Biogeochemistry of tube-dwellings: A study of the sedentary polychaete Amphitrite ornata (Leidy)

Most studies of near interface sediments assume that chemically and biologicalJy important properties are stratified vertically in a deposit. Sampling patterns reflect this assumption and little attention has been paid to three-dimensional heterogeneity. In this study the effects of burrow structures formed by Amphitrite ornata on the distribution of physical, chemical, and biological characteristics of a deposit are investigated...

Seasonal, 2-D sedimentary extracellular enzyme activities and controlling processes in Great Peconic Bay, Long Island

Extracellular enzymes (EE) initiate heterotrophic remineralization by hydrolyzing high-molecularweight organic matter to substrates that are sufficiently small (approximately 600 Da) to be transported across cell membranes. An accurate understanding of EE associated remineralization processes in sedimentary deposits requires measuring patterns of extracellular enzyme activity (EEA) with minimal disturbance of natural sediment structure. In this study, two-dimensional patterns of extracellular enzyme (leucine aminopeptidase) activity in shallow-water, marine sediments from Great Peconic Bay, Long Island, New York were examined seasonally at sub-millimeter resolution by using a newly developed EE planar fluorosensor. Comparisons of spatially averaged, vertical enzyme activity profiles measured using this imaging sensor system and traditional sediment homogenization techniques verified the overall consistency of the methodology. The depth-averaged EEA (approximately 10 cm) varied seasonally with highest levels in the late spring through summer (0.2 μmol substrate g-wet-wt–1 hr–1) and lowest in the late fall and early winter (0.1 μmol substrate g-wet-wt–1 hr–1). EEA distributions, however, showed extensive small-scale horizontal heterogeneity as well as vertical variations. Both the input of reactive substrates (planktonic organic matter) and temperature differences accounted for major changes in EEA seasonally. In general, horizontal heterogeneity in EEA was greatest during warm seasons (summer, fall) as a result of increased macrofaunal activity. On the other hand, vertical variations are less significant during warm periods compared with cold periods as the sediment is more intensely reworked. Hot spots of elevated microbial activity from sub-millimeter to millimeter scales are observed in some seasons and are specifically associated with substrate inputs from phytoplankton blooms and particle reworking by infauna. The deposition of phytodetritus from an early spring bloom greatly enhanced surface sediment EEA, and at this time high EEA closely coincided with regions of elevated metabolite production as measured by NH+4 and ΣCO2 concentrations. Direct correlations between averaged EEA distributions and nutrient production rates were observed throughout the year but no correlations between EEA and pore water nutrient concentrations were present. Spatially resolved EEA directly tracks reactive particle distributions and is generally independent of solute transport mechanisms, such as bioirrigation, and redox conditions

Surficial bioturbation and rapid benthic remineralization in the Cape Hatteras shelf/slope region. Final report

This is a final report for the DOE of grant DE-FG02-92ER61464 ''Surficial bioturbation and rapid benthic remineralization in the Cape Hatteras shelf slope region''. Over the past 6 years we have participated in a multidisciplinary field study called the Ocean margins Program (OMP) to examine the importance of continental margins in the global carbon cycle. Specifically, we have focused on the southern portion of the Mid-Atlantic Bight between Cape Hatteras and Chesapeake Bay where a large flux of freshwater and organic carbon enters the North Atlantic Ocean. Additionally, during the first stage of this project, we developed the use of CM-a distributions in sediments as a quantitative indicator of benthic C flux and remineralization rates. The primary objective of our research group has been to understand mechanisms and quantify biogeochemical processes in the seabed that affect cycling, flux, and storage of carbon on the ocean margin of the Mid-Atlantic Bight

The Codevelopment of Mangroves and Infaunal Community Diversity in Response to the Natural Dynamics of Mud Deposition in French Guiana

The sustainability of mangrove ecosystems requires a knowledge of their spatiotemporal variability as a function of regional properties. The unique coastal ecosystems of the mangrove belt along the coast of the Guianas in South America are influenced by cycles of a massive accretion of mud supplied by the Amazon River and wave induced erosion. This study characterized, for the first time, how benthic infaunal assemblages, as proxies of mechanisms of mangrove resilience, were structured by the natural growth track of Avicennia germinans dominated mangroves in French Guiana. We sampled 4 mobile mud stations and 27 consolidated mud stations distributed over 9 tidal transects from bare to vegetated mudflats colonized by young mangroves during the dry season. We collected a complete dataset of sediment and vegetation variables together with the benthic meso- (>0.25 mm) and macrofauna (>1 mm). We used a combination of eigenvector based multivariate analyses and variance partitioning on this multiple set of variables to identify which environmental variables likely drive the benthic diversity patterns. Mangrove early development increased the alpha and beta diversities of the infaunal communities for the two size classes. A total of 20–30% and 7–12% of the beta diversity are explained by linear and nonlinear spatial variables, respectively. However, 7% to 9% of the variance partioning could be determined by other biotic/abiotic variables, biological interactions or neutral processes, not described here. This study has highlighted the necessity of taking into account mangrove dynamics at suitable spatial scales for benthic biodiversity evaluation and mangrove management or restoration plan

Micro-spectroscopic and freezing characterization of ice-nucleating particles collected in the marine boundary layer in the eastern North Atlantic

Formation of atmospheric ice plays a crucial role in the microphysical evolution of mixed-phase and cirrus clouds and thus climate. How aerosol particles impact ice crystal formation by acting as ice-nucleating particles (INPs) is a subject of intense research activities. To improve understanding of atmospheric INPs, we examined daytime and nighttime particles collected during the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) field campaign conducted in summer 2017. Collected particles, representative of a remote marine environment, were investigated for their propensity to serve as INPs in the immersion freezing (IMF) and deposition ice nucleation (DIN) modes. The particle population was characterized by chemical imaging techniques such as computer-controlled scanning electron microscopy with energy-dispersive X-ray analysis (CCSEM/EDX) and scanning transmission X-ray microscopy with near-edge X-ray absorption fine-structure spectroscopy (STXM/NEXAFS). Four major particle-type classes were identified where internally mixed inorganic-organic particles make up the majority of the analyzed particles. Following ice nucleation experiments, individual INPs were identified and characterized by SEM/EDX. The identified INP types belong to the major particle-type classes consisting of fresh sea salt with organics or processed sea salt containing dust and sulfur with organics. Ice nucleation experiments show IMF events at temperatures as low as 231 K, including the subsaturated regime. DIN events were observed at lower temperatures of 210 to 231 K. IMF and DIN observations were analyzed with regard to activated INP fraction, ice-nucleation active site (INAS) densities, and a water activity-based immersion freezing model (ABIFM) yielding heterogeneous ice nucleation rate coefficients. Observed IMF and DIN events of ice formation and corresponding derived freezing rates demonstrate that the marine boundary layer aerosol particles can serve as INPs under typical mixed-phase and cirrus cloud conditions. The derived IMF and DIN parameterizations allow for implementation in cloud and climate models to evaluate predictive effects of atmospheric ice crystal formation