The release of phosphorus from sediments can stimulate algal blooms in eutrophic water bodies worldwide. One technique to reduce this release involves capping the sediment with millimetre-thick layers of chemically active materials such as aluminium hydroxide floc (alum), and the mineral-based products Phoslock and modified zeolite. The effects of this technique on transport and reaction of diagenetically important sediment compounds other than phosphorus are unknown. The present study used microelectrodes to measure the apparent gas diffusivity of capping layers derived from different doses of these capping materials and their effects on pore water pH and dissolved molecular oxygen. The apparent O2 diffusivity of alum capping layers (1.58 × 10–5 cm2 s–1) was constant with depth and higher than that of mineral-based capping layers (~1.15 × 10–5 cm2 s–1 and decreasing with depth in the capping layer). The capping materials raised the depth of the oxic–anoxic interface and associated pH minimum and altered the sediment O2 consumption as functions of the capping-layer thickness and apparent diffusivity. Modified zeolite layers decreased pore water pH slightly (0.3–0.5 units); alum layers decreased pH by 1–2.2 units. It is proposed that capping layers derived from doses >200 g m–2 can alter benthic process rates and solute fluxes.

Underwater optical wireless communication (UOWC) is a field of research that has gained popularity with the development of unmanned underwater vehicle (UUV) technologies. Its utilization is crucial in offshore industries engaging in sustainable alternatives for food production and energy security. Although UOWC can meet the high data rate and low latency requirements of underwater video transmission for UUV operations, the links that enable such communication are affected by the inhomogeneous light attenuation and the presence of sunlight. Here, we present how the underwater spectral distribution of the light field can be modeled along the depths of eight stratified oceanic water types. We considered other established models, such as SPCTRL2, Haltrin’s single parameter model for inherent optical properties, and a model for the estimation of the depth distribution of chlorophyll-a, and present insights based on transmission wavelength for the maximum signal-to-noise ratio (SNR) under different optical link parameter combinations such as beam divergence and transmit power under “daytime” and “nighttime” conditions. The results seem to challenge the common notion that the blue-green spectrum is the most suitable for underwater optical communication. We highlight a unique relationship between the transmission wavelength for the optimal SNR and the link parameters and distance, which varies with depth depending on the type of oceanic water stratification. Our analyses further highlighted potential implications for solar discriminatory approaches and strategies for routing in cooperative optical wireless networks in the photic region.

Ocean acidification may alter the cycling of nitrogen in coastal sediment and so the sediment–seawater nitrogen flux, an important driver of pelagic productivity. To investigate how this perturbation affects the fluxes of NO_X⁻ (nitrite/nitrate), NH₄⁺ and O₂, we incubated estuarine sand and subtidal silt in recirculating seawater with a CO₂-adjusted pH of 8.1 and 7.9. During a 41-day incubation, the seawater kept at pH 8.1 lost 97% of its NO_X⁻ content but the seawater kept at pH 7.9 lost only 18%. Excess CO₂ increased benthic photosynthesis. In the silt, this was accompanied by a reversal of the initial NO_X⁻ efflux into influx. The estuarine sand sustained its initial NO_X⁻ influx but, by the end of the incubation, released more NH₄⁺ at pH 7.9 than at pH 8.1. We hypothesise that these effects share a common cause; excess CO₂ increased the growth of benthic microalgae and so nutrient competition with ammonia oxidising bacteria (AOB). In the silt, diatoms likely outcompeted AOB for NH₄⁺ and photosynthesis increased the dark/light fluctuations in the pore water oxygenation inhibiting nitrification and coupled nitrification/denitrification. If this is correct, then excess CO₂ may lead to retention of inorganic nitrogen adding to the pressures of increasing coastal eutrophication.

Underwater optical wireless communication (UOWC) has gained interest in recent years with the introduction of autonomous and remotely operated mobile systems in blue economic ventures such as offshore food production and energy generation. Here, we devised a model for estimating the received power distribution of diffused line-of-sight mobile optical links, accommodating irregular intensity distributions beyond the beam-spread angle of the emitter. We then used this model to conduct a spatial analysis investigating the parametric influence of the placement, orientation, and angular spread of photodiodes in array-based receivers on the mobile UOWC links in different Jerlov seawater types. It revealed that flat arrays were best for links where strict alignment could be maintained, whereas curved arrays performed better spatially but were not always optimal. Furthermore, utilizing two or more spectrally distinct wavelengths and more bandwidth-efficient modulation may be preferred for received-signal intensity-based localization and improving link range in clearer oceans, respectively. Considering the geometric implications of the array of receiver photodiodes for mobile UOWCs, we recommend the use of dynamically shape-shifting array geometries.

Ecosystem feedbacks in response to ocean acidification can amplify or diminish diel pH oscillations in productive coastal waters. Benthic microalgae generate such oscillations in sediment porewater and here we ask how CO₂ enrichment (acidification) of the overlying seawater alters these in the absence and presence of biogenic calcite. We placed a 1-mm layer of ground oyster shells, mimicking the arrival of dead calcifying biota (+Calcite), or sand (Control) onto intact silt sediment cores, and then gradually increased the pCO₂ in the seawater above half of +Calcite and Control cores from 472 to 1216 μatm (pH 8.0 to 7.6, CO₂:HCO₃⁻ from 4.8 to 9.6 × 10⁻⁴). Porewater [O₂] and [H⁺] microprofiles measured 16 d later showed that this enrichment had decreased the O₂ penetration depth (O₂-pd) in +Calcite and Control, indicating a metabolic response. In CO₂-enriched seawater: (1) sediment biogeochemical processes respectively added and removed more H⁺ to and from the sediment porewater in darkness and light, than in ambient seawater increasing the amplitude of the diel porewater [H⁺] oscillations, and (2) in darkness, calcite dissolution in +Calcite sediment decreased the porewater [H⁺] below that in overlying seawater, reversing the sediment–seawater H⁺ flux and decreasing the amplitude of diel [H⁺] oscillations. This dissolution did not, however, counter the negative effect of CO₂ enrichment on O₂-pd. We now hypothesise that feedback to CO₂ enrichment—an increase in the microbial reoxidation of reduced solutes with O₂—decreased the sediment O₂-pd and contributed to the enhanced porewater acidification.