Nitrogen cycling in muddy sediments of Great Peconic Bay, USA: Seasonal N reaction balances and multi-year flux patterns

To better understand the capacity of sediments to serve as both source and sink of nitrogen (N) and to identify any evidence of evolving changes in sedimentary N cycling, N2 production, N remineralization, and N2 fixation were studied over a multi-year period (2010–2015) in bioturbated mud of Great Peconic Bay, a temperate northeastern U. S. estuary. Benthic fluxes and rates of organic matter remineralization were measured using in situ and ex situ incubations. Net annual NH+ 4, NO–3/NO–2, and N2–N fluxes (μ = 1.1, 0.03, and 1.2 mmol m –2d –1) were close to averages for comparable sedi- mentary environments from surveys of published field studies. Net N2 fluxes (by membrane inlet mass spectrometry) were influenced in different periods by temperature, oxygenation of sediment, pulsed Corg, and the activity of benthic macrofauna and benthic microalgae, although no single physical or biogeochemical variable showed a strong, direct relationship with net N2 fluxes over all sampling periods. In situ measurements sometimes showed more dynamic and higher amplitude diurnal N flux cycles than did ex situ incubations, suggesting ex situ incubations did not fully capture impacts of bioirrigation or benthic photosynthesis.15 N tracer experiments indicated anammox was < 7% of total N2 production. Acetylene reduction assays demonstrated C2 H4 production to depths ≥ 15 cm and suggested N2 fixation may have approached 25% of gross N2 production(3:1 C2 H4 : N2). Mass balances incorporating independently measured N remineralization estimates were consistent with measured levels of N2 fixation. Overall, complex balances of competing processes governed sedimentary N cycling seasonally, and N2 production dominated N2 fixation. Measured N2 fixation was consistent with constraints from N remineralization rates and net N fluxes except in episodic conditions (e. g., algal blooms). There was no indication of progressive changes in N cycling magnitudes or relative N reaction balances over the study period.

Density and size-dependent bioturbation effects of the infaunal polychaete Nephtys incisa on sediment biogeochemistry and solute exchange

The impact of bioturbation on the geochemistry of aquatic sediments is known to depend on the benthic infauna species that are present. However, burrowing and activity patterns of each species may also change during the different stages of a life cycle. In this study, we examined the effects of four size classes of the polychaete Nephtys incisa on burrow networks and sediment biogeochemistry. In our experimental aquaria, the total biovolume (~ biomass) of Nephtys was kept constant, but different age classes were introduced, so the size and abundance varied between treatments. Despite differences in the geometry of burrow networks (due to varying density and size of burrows as revealed by X-radiography), the transport of nonreactive solutes (Br–) showed little difference between treatments. In contrast, the depth distribution of reactive solutes (Fe2+, Mn2+, TPO3– 4, TCO2, O2, pH) depended on oxidized sediment volumes and on spatial micro-heterogeneity related to burrowing patterns. Net fluxes of O2, TCO2, and NO– 3 fluxes were strongly affected by age-dependent burrowing patterns. Carbonate dissolution and remineralization rates (reflected by TCO 2fluxes) were enhanced as the size of individuals increased. NO– 3fluxes showed progressive change from dominance of nitrification (release) to denitrification (uptake) as burrow densities decreased with larger individuals. We conclude that different age-size classes of a single species at identical biovolume affect biogeo- chemical cycling differently, due to changes in burrow sizes and burrow densities. Because of redox reaction coupling associated with burrow geometries (Fe2+, Mn2+ oxidation patterns), similar magnitudes of nonlocal transport may be a misleading indicator of biogenic impacts. Our observations demonstrate that biogeochemical impacts must be evaluated in the context of size (age-) specific traits and population densities rather than biomass or biovolume alone.

A bathythermograph

Originally published April 9, 1938, in the Journal of Marine Research 1(2), 95???100.

On properties of seawater defined by temperature, salinity, and pressure

Hydrographic station data, consisting principally of temperature and salinity determinations, have been used by physical oceanographers to develop a climatological picture of the distribution of these quantities in the oceans of the world. Density as determined by Knudsen's formula, taken together with hydrostatic and geostrophic dynamics, also provides a crude picture of oceanic flow. However, the data probably contain substantially more information than has been derived from them in the past.The quantity that is orthogonal to potential-density curves in the S plane is suggested as a useful variable to complement the information contained in potential density. The derivation of this quantity, denoted by τ in this paper, is straightforward. A polynomial expression for τ that is suitable for computer calculations of τ from hydrographic station data is given. Shown are examples of hydrographic station data from the Atlantic plotted on the τσ diagram. The information contained in the τσ diagram shows many of the features exhibited in the TS plane. Vertical sections of τ appear to provide information about mixing in different parts of the Atlantic. The distribution of τ for abyssal waters at selected stations in the oceans of the world resembles the distribution of abyssal density as plotted by Lynn and Reid (1968). From the data presented, it appears that τ may serve as a good tracer for abyssal water movements.Since τ is defined to be orthogonal to σ, the expectation is that τ is a dynamically passive variable. However, since σ does not correlate with abyssal densities, it appears to lose dynamical significance at great depth, and τ assumes dynamical significance because of its orthogonality to σ. This unexpected feature leads to an exploration of the dynamical significance of σ. A natural starting point is the question of stability of abyssal water.A distinction is made between stability as determined by in situ determinations and as determined by the potential-density (σ) distribution. Simple examples are presented to show that analysis based on σ alone can lead to incorrect conclusions about gravitational stability of the water in the abyssal ocean. The reason is that seawater is a multicomponent thermodynamic system, and the thermodynamic coefficients are functions of pressure, salinity, and temperature. This functional dependence leads to adjustments in density as a water particle moves adiabatically in the vertical direction so that a layer of water that appears to be unstable near the surface may be stable (as determined by in situ determination) at great depth. A local potential density, which is simply the vertical integral of the in situ stability, is derived. This quantity gives a precise picture of gravitational stability in the vertical direction. Some distributions of local potential density are shown.Originally published May 15, 1972, in the Journal of Marine Research 30(2), 227???255.

On the process of upwelling

Originally published April 9, 1938, in the Journal of Marine Research 1(2), 155???164

Gravitational circulation in straits and estuaries

A coupled set of partial differential equations and associated boundary conditions is written to describe circulation and salt-flux processes for estuaries in which turbulent mixing results primarily from tidal currents. Similarity solutions, motivated by characteristic salinity distributions observed in estuaries, are obtained for this set of equations and are compared with observational data. The circulation is separated into modes analogous to the barotropic, baroclinic, and Ekman modes of oceanic circulation. The salinity distribution, although coupled to the velocity distribution, is found to vary independently of it as well. The theoretical results are discussed in regard to: (i) correlation between the vertical variations of mean velocity and salinity, (ii) the role of this correlation in maintaining the steady-state salinity distribution in estuaries, and (iii) some implications for computations of flushing and dispersion of contaminants.