Submarine Groundwater Discharge Exceeds River Inputs as a Source of Nutrients to the Great Barrier Reef.

Rivers are often assumed to be the main source of nutrients triggering eutrophication in the Great Barrier Reef (GBR). However, existing nutrient budgets suggest a major missing source of nitrogen and phosphorus sustaining primary production. Here, we used radium isotopes to resolve submarine groundwater discharge (SGD)-derived, shelf-scale nutrient inputs to the GBR. The total SGD was ∼10-15 times greater than average river inputs, with nearshore groundwater discharge accounting for ∼30% of this. Total SGD accounted for >30% of all known dissolved inorganic N and >60% of inorganic P inputs and exceeded regional river inputs. However, SGD was only a small proportion of the nutrients necessary to sustain primary productivity, suggesting that internal recycling processes still dominate the nutrient budget. With millions of dollars spent managing surface water nutrient inputs to reef systems globally, we argue for a shift in the focus of management to safeguard reefs from the impacts of excess nutrients.

Compound climate extremes driving recent sub-continental tree mortality in northern Australia have no precedent in recent centuries.

Compound climate extremes (CCEs) can have significant and persistent environmental impacts on ecosystems. However, knowledge of the occurrence of CCEs beyond the past ~ 50 years, and hence their ecological impacts, is limited. Here, we place the widespread 2015-16 mangrove dieback and the more recent 2020 inland native forest dieback events in northern Australia into a longer historical context using locally relevant palaeoclimate records. Over recent centuries, multiple occurrences of analogous antecedent and coincident climate conditions associated with the mangrove dieback event were identified in this compilation. However, rising sea level-a key antecedent condition-over the three decades prior to the mangrove dieback is unprecedented in the past 220 years. Similarly, dieback in inland forests and savannas was associated with a multi-decadal wetting trend followed by the longest and most intense drought conditions of the past 250 years, coupled with rising temperatures. While many ecological communities may have experienced CCEs in past centuries, the addition of new environmental stressors associated with varying aspects of global change may exceed their thresholds of resilience. Palaeoclimate compilations provide the much-needed longer term context to better assess frequency and changes in some types of CCEs and their environmental impacts.

Are methane emissions from mangrove stems a cryptic carbon loss pathway? Insights from a catastrophic forest mortality.

Growing evidence indicates that tree-stem methane (CH4 ) emissions may be an important and unaccounted-for component of local, regional and global carbon (C) budgets. Studies to date have focused on upland and freshwater swamp-forests; however, no data on tree-stem fluxes from estuarine species currently exist. Here we provide the first-ever mangrove tree-stem CH4 flux measurements from  >50 trees (n = 230 measurements), in both standing dead and living forest, from a region suffering a recent large-scale climate-driven dieback event (Gulf of Carpentaria, Australia). Average CH4 emissions from standing dead mangrove tree-stems was 249.2 ± 41.0 µmol m-2  d-1 and was eight-fold higher than from living mangrove tree-stems (37.5 ± 5.8 µmol m-2  d-1 ). The average CH4 flux from tree-stem bases (c. 10 cm aboveground) was 1071.1 ± 210.4 and 96.8 ± 27.7 µmol m-2  d-1 from dead and living stands respectively. Sediment CH4 fluxes and redox potentials did not differ significantly between living and dead stands. Our results suggest both dead and living tree-stems act as CH4 conduits to the atmosphere, bypassing potential sedimentary oxidation processes. Although large uncertainties exist when upscaling data from small-scale temporal measurements, we estimated that dead mangrove tree-stem emissions may account for c. 26% of the net ecosystem CH4 flux.

Do invasive corals alter coral reef processes? An empirical approach evaluating reef fish trophic interactions.

Understanding how invasive species affect key ecological interactions and ecosystem processes is imperative for the management of invasions. We evaluated the effects of invasive corals (Tubastraea spp.) on fish trophic interactions in an Atlantic coral reef. Remote underwater video cameras were used to examine fish foraging activity (bite rates and food preferences) on invasive cover levels. Using a model selection approach, we found that fish feeding rates declined with increased invasive cover. For Roving Herbivores (RH) and Sessile Invertivores (SI), an abrupt reduction of fish feeding rates corresponded with higher invasive cover, while feeding rates of Territorial Herbivores (TH) and Mobile Invertivores (MI) decreased linearly with cover increase. Additionally, some fish trophic groups, such as RH, SI and Omnivores (OM), had lower densities in reef sections with high invasive cover. These findings demonstrate that invasive corals negatively impact fish-benthic interactions, and could potentially alter existing trophic relationships in reef ecosystems.

Pristine mangrove creek waters are a sink of nitrous oxide.

Nitrous oxide (N2O) is an important greenhouse gas, but large uncertainties remain in global budgets. Mangroves are thought to be a source of N2O to the atmosphere in spite of the limited available data. Here we report high resolution time series observations in pristine Australian mangroves along a broad latitudinal gradient to assess the potential role of mangroves in global N2O budgets. Surprisingly, five out of six creeks were under-saturated in dissolved N2O, demonstrating mangrove creek waters were a sink for atmospheric N2O. Air-water flux estimates showed an uptake of 1.52 ± 0.17 µmol m(-2) d(-1), while an independent mass balance revealed an average sink of 1.05 ± 0.59 µmol m(-2) d(-1). If these results can be upscaled to the global mangrove area, the N2O sink (~2.0 × 10(8) mol yr(-1)) would offset ~6% of the estimated global riverine N2O source. Our observations contrast previous estimates based on soil fluxes or mangrove waters influenced by upstream freshwater inputs. We suggest that the lack of available nitrogen in pristine mangroves favours N2O consumption. Widespread and growing coastal eutrophication may change mangrove waters from a sink to a source of N2O to the atmosphere, representing a positive feedback to climate change.