Intertidal zones are spatially complex and temporally dynamic environments. Coastal groundwater discharge, including submarine groundwater discharge, may provide stabilizing conditions for intertidal ...zone permeable sediments. In this study, we integrated detailed time series temperature observations, porewater pressure measurements, and two‐dimensional electrical resistivity tomography profiles to understand the coupled hydraulic‐thermal regime of a tropical sandy intertidal zone in a fringing coral reef lagoon (Rarotonga, Cook Islands). We found three heating patterns across the 15 m study transect over tidal and diel periods: (1) a highly variable thermal regime dominated by swash infiltration and changes in saturation state in the upper foreshore with net heat import into the sediment, (2) a groundwater‐supported underground stable, cool region just seaward of the intertidal slope break also importing heat into the subsurface, and (3) a zone of seawater recirculation that sustained consistently warm subsurface temperatures that exported heat across the sediment‐water interface. Simple calculations suggested thermal conduction as the main heat transport mechanism for the shallow intertidal sediment, but deeper and/or multidimensional groundwater flow was required to explain temperature patterns beyond 20 cm depth. Temperature differences between the distinct hydrodynamic zones of the foreshore site resulted in significant thermal gradients that persisted beyond tidal and diel periods. The thermal buffering of intertidal zones by coastal groundwater systems, both at surface seeps and in the shallow subsurface, can be responsible for thermal refugia for some coastal organisms and hotspots for biogeochemical reactions.
Key Points
We observed coupled fluid flow and heat transport in intertidal sediment
Data suggest thermal zonation due to different groundwater mixing regimes
Terrestrially‐sourced groundwater buffers thermal variability and forms refugia
Seasonal rates of benthic gross primary production, net primary production and respiration were measured and whole-system carbon budgets constructed in 3 subtropical estuaries with different ...catchment land-use intensities to better understand how land-use changes influence benthic metabolism. Annual benthic net ecosystem metabolism (NEM) indicates that systems become more heterotrophic with increasing land-use intensity. This is due to a combination of an increase in the area of unvegetated habitats and the unvegetated habitats becoming more heterotrophic with increasing land-use intensity. Whole-system NEM is closely linked to benthic NEM, highlighting the important control of benthic metabolism on whole-system metabolism in shallow coastal systems. Carbon mass balances show whole-system net metabolism also shifted from net autotrophic to net heterotrophic, with a concomitant switch from CO₂ uptake to emission, with increasing land-use intensity. Our findings demonstrate that land-use changes shift wholeestuary metabolism by altering both habitat distribution and within-habitat metabolism rates.
A mass coral spawning event on the Heron Island reef flat in 2005 provided a unique opportunity to examine the response of a coral reef ecosystem to a large episodic nutrient addition. A post-major ...spawning phytoplankton bloom resulted in only a small drawdown of dissolved inorganic phosphorus (DIP minimum = 0.37 micromol L⁻¹), compared with almost complete removal of dissolved inorganic nitrogen (DIN) (minimum NO⁻₃ =0.01 micromol L⁻¹; NH⁺₄ = 0.11 micromol L⁻¹), suggesting that pelagic primary production is potentially N limited on the timescale of this study. DIN, DIP, dissolved organic nitrogen (DON), and dissolved organic phosphorus were used in the production of biomass, and mass balance calculations highlighted the importance of organic forms of N and P for benthic and pelagic production in tropical coral reef environments characterized by low inorganic N and P. The input of N and P via the deposition of coral spawn and associated phytodetritus resulted in large changes to N cycling in the sediments, but only small changes to P cycling, because of the buffering capacity provided by the large pool of bioavailable P. It is most likely that this large pool of bioavailable P in the sediments drives potential N limitation of benthic coral reef communities. For example, there was sufficient bioavailable P stored in the top 10 cm of the sediment column to sustain the prespawning rates of benthic production for over 200 d. Most of the change in benthic N cycling occurred via DON and N₂ pathways, driven by changes in the quantity and quality of organic matter deposited and decomposed post-major spawning. The heterotrophic and autotrophic microbial communities within the coral reef sands were able to rapidly (6 to 7 d) process the large episodic load of N and P provided by coral mass spawning.
Global warming (and the consequent increase in sea surface temperature) is expected to modify rates of gross primary production (GPP), respiration (
R
), and net calcium carbonate (CaCO
3
) ...dissolution in permeable coral reef carbonate sediments. Previous simulations of seawater warming on coral reef sediments found a decline in the GPP/
R
ratio and an associated increase in CaCO
3
dissolution but were only conducted over a short timescale (< 24 h). To date, no studies have examined the prolonged (> 24 h) effect of seawater warming on coral reef CaCO
3
sediment metabolism and dissolution, which may allow the benthic community to acclimatise. This study used 600-L flume aquaria to examine the effect of seawater warming on GPP,
R
, and CaCO
3
dissolution in the permeable coral reef CaCO
3
sediments of Mo’orea, French Polynesia, over a period of 15 d. On average, when exposed to warmed seawater (+ 2.8 °C),
R
in the CaCO
3
sediments was enhanced (+ 58%) to a greater extent than GPP (+19%), resulting in a decline in GPP/
R
(− 23%) and an associated increase in net CaCO
3
dissolution (+ 126%). The magnitude of these warming-mediated metabolic changes increased each day until reaching a plateau after about 8 d, indicating that 24-h experiments may be underestimating the effect of warming over longer timescales. Interestingly, the increase in dissolution relative to control treatments was more striking during the day (+ 163%) than at night (+ 89%), suggesting that warming acted to both enhance geochemical dissolution and reduce biogenic calcification or inorganic precipitation. Together, these data indicate that, over the timescale observed here, photosynthesis and associated inorganic and biogenic CaCO
3
precipitation do not exhibit the ability to counterbalance the warming-mediated increase in sediment heterotrophy and CaCO
3
dissolution.
Tropical peat-draining rivers are known as potentially large sources of carbon dioxide (CO2) to the atmosphere due to the high loads of carbon they receive from surrounding soils. However, not many ...seasonally resolved data are available, limiting our understanding of these systems. We report the first measurements of carbon dioxide partial pressure (pCO2) in the Rajang River and Estuary, the longest river in Malaysia. The Rajang River catchment is characterized by extensive peat deposits found in the delta region, and by human impact such as logging, land use and river damming. pCO2 averaged 2540±189 µatm during the wet season and 2350±301 µatm during the dry season. Using three different parameterizations for the gas transfer velocity, calculated CO2 fluxes to the atmosphere were 1.5 (0.5–2.0) g C m−2 d−1 (mean, minimum – maximum) during the wet season and 1.7 (0.6–2.6) g C m−2 d−1 during the dry season. This is at the low end of reported values for Southeast Asian peat-draining rivers, but similar to values reported for Southeast Asian rivers that do not flow through peat deposits. In the Rajang River, peatlands probably do not contribute much to the CO2 flux due to the proximity of the peatlands to the coast, which limits the opportunity for degradation of organic C during transport. Thus, we suggest that peat coverage is, by itself, insufficient as the sole predictor of CO2 emissions from peat-draining rivers, and that other factors, like the spatial distribution of peat in the catchment and pH, also need to be considered.
Globally, coral reefs are threatened by ocean warming and acidification. The degree to which acidification will impact reefs is dependent on the local hydrodynamics, benthic community composition, ...and biogeochemical processes, all of which vary on different temporal and spatial scales. Characterizing the natural spatiotemporal variability of seawater carbonate chemistry across different reefs is critical for elucidating future impacts on coral reefs. To date, most studies have focused on select habitats, whereas fewer studies have focused on reef scale variability. Here, we investigate the temporal and spatial seawater physicochemical variability across the entire Heron Island coral reef platform, Great Barrier Reef, Australia, for a limited duration of six days. Autonomous sensor measurements at three sites across the platform were complemented by reef-wide boat surveys and discrete sampling of seawater carbonate chemistry during the morning and evening. Variability in both temporal and spatial physicochemical properties were predominantly driven by solar irradiance (and its effect on biological activity) and the semidiurnal tidal cycles but were influenced by the local geomorphology resulting in isolation of the platform during low tide and rapid flooding during rising tides. As a result, seawater from previous tidal cycles was sometimes trapped in different parts of the reef leading to unexpected biogeochemical trends in space and time. This study illustrates the differences and limitations of data obtained from high-frequency measurements in a few locations compared to low-frequency measurements at high spatial resolution and coverage, showing the need for a combined approach to develop predictive capability of seawater physicochemical properties on coral reefs.
Denitrification efficiency DE; (N₂ - N/(DIN + N₂ - N) x 100%) as an indicator of change associated with nutrient over-enrichment was evaluated for 22 shallow coastal ecosystems in Australia. The rate ...of carbon decomposition (which can be considered a proxy for carbon loading) is an important control on the efficiency with which coastal sediments in depositional mud basins with low water column nitrate concentrations recycle nitrogen as N₂. The relationship between DE and carbon loading is due to changes in carbon and nitrate (NO₃) supply associated with sediment biocomplexity. At the DE optimum (500-1,000 μmol m⁻² h⁻¹), there is an overlap of aerobic and anaerobic respiration zones (caused primarily by the existence of anaerobic micro-niches within the oxic zone, and oxidized burrow structures penetrating into the anaerobic zone), which enhances denitrification by improving both the organic carbon and nitrate supply to denitrifiers. On either side of the DE optimum zone, there is a reduction in denitrification sites as the sediment loses its three-dimensional complexity. At low organic carbon loadings, a thick oxic zone with low macrofauna biomass exists, resulting in limited anoxic sites for denitrification, and at high carbon loadings, there is a thick anoxic zone and a resultant lack of oxygen for nitrification and associated NO₃ production. We propose a trophic scheme for defining critical (sustainable) carbon loading rates and possible thresholds for shallow coastal ecosystems based on the relationship between denitrification efficiency and carbon loading for 17 of the 22 Australian coastal ecosystems. The denitrification efficiency “optimum” occurs between carbon loadings of about 50 and 100 g C m⁻² year⁻¹. Coastal managers can use this simple trophic scheme to classify the current state of their shallow coastal ecosystems and for determining what carbon loading rate is necessary to achieve any future state.
On high-energy rocky shores receiving treated wastewater, impacts are difficult to distinguish against a highly variable background and are localised due to rapid dilution. We demonstrate that ...nitrogen stable isotope values (δ15N) of rocky shore biota are highly sensitive to wastewater inputs. For macroalgae (Ulva lactuca and Endarachne binghamiae), grazing snails (Bembicium nanum and Nerita atramentosa), and predatory snails (Morula marginalba), δ15N was enriched near a wastewater outfall and declined with distance, returning to background levels within 290m. Any of these species therefore indicates the extent of influence of wastewater, allowing identification of an appropriate scale for studies of ecosystem impacts. For M. marginalba, significant regressions between δ15N and tissue copper, manganese, and zinc concentrations indicate a possible wastewater source for these metals. This suggests that δ15N is a proxy for exposure to wastewater contaminants, and may help to attribute variations in rocky shore communities to wastewater impacts.
•N stable isotopes are sensitive tracers of wastewater in high-energy environments.•N uptake by rocky shore biota was very localised (within 290m of an outfall).•Macroalgae, grazing snails, and predatory snails showed similar N uptake patterns.•δ15N values correlated with some metal concentrations for some biota.•Wastewater impacts on biota may be identified using δ15N as a proxy for exposure.
Dimethylsulfoniopropionate (DMSP) and eleven other target zwitterions were quantified in the branch tips of six
Acropora
species and
Stylophora pistillata
hard coral growing on the reef flat ...surrounding Heron Island in the southern Great Barrier Reef (GBR), Australia. Hydrophilic interaction liquid chromatography mass spectrometry (HILIC-MS) was used for sample analysis with isotope dilution MS applied to quantify DMSP. The concentration of DMSP was ten times greater in
A. aspera
than
A. valida
, with this difference being maintained throughout the spring, summer and winter seasons. In contrast, glycine betaine was present in significantly higher concentrations in these species during the summer than the winter. Exposure of branch tips of
A. aspera
to air and hypo-saline seawater for up to 1 h did not alter the concentrations of DMSP present in the coral when compared with control samples. DMSP was the most abundant target zwitterion in the six
Acropora
species examined, ranging from 44-78% of all target zwitterions in
A. millepora
and
A. aspera
, respectively. In contrast, DMSP only accounted for 7% in
S. pistillata
, with glycine betaine and stachydrine collectively accounting for 88% of all target zwitterions in this species. The abundance of DMSP in the six
Acropora
species examined points to
Acropora
coral being an important source for the biogeochemical cycling of sulfur throughout the GBR, since this reef-building branching coral dominates the coral cover of the GBR.
Graphical Abstract
HILIC-MS extracted ion chromatogram showing zwitterionic metabolites from the branching coral
Acropora isopora
Temperature, light and carbonate chemistry speciation all influence the growth, calcification and photosynthetic carbon fixation rates of coccolithophores to a similar degree. There have been ...multiple attempts to project the responses of coccolithophores to changes in carbonate chemistry, but the interaction with light and temperature remains elusive. Here we devise a simple conceptual model to derive a fit equation for coccolithophorid growth, photosynthetic carbon fixation and calcification rates in response to simultaneous changes in carbonate chemistry speciation, temperature and light conditions. The fit equation is able to account for up to 88% of the variability in measured metabolic rates. Equation projections indicate that temperature, light and carbonate chemistry speciation all have different modulating effects on both optimal growth conditions and the sensitivity of responses to extreme environmental conditions. Calculations suggest that a single extreme environmental condition (CO2, temperature, light) will reduce maximum rates regardless of how optimal the other environmental conditions may be. Thus, while the response of coccolithophores to ocean change depends on multiple variables, the one which is least optimal will have the most impact on overall rates. Finally, responses to ocean change are usually reported in terms of cellular rates. However, changes in cellular rates can be a poor predictor for assessing changes in production at the community level. We therefore introduce a new metric, the calcium carbonate production potential (CCPP), which combines the independent effects of changes in growth rate and cellular calcium carbonate content to assess how environmental changes will impact coccolith production. Direct comparison of CO2 impacts on cellular CaCO3 production rates and CCPP shows that while the former is still at 45% of its pre-industrial capacity at 1000 uatm, the latter is reduced to 10%.
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