Alistair Grinham

Interactions between sea-level rise and wave exposure on reef island dynamics in the Solomon Islands

Low-lying reef islands in the Solomon Islands provide a valuable window into the future impacts of global sea-level rise. Sea-level rise has been predicted to cause widespread erosion and inundation of low-lying atolls in the central Pacific. However, the limited research on reef islands in the western Pacific indicates the majority of shoreline changes and inundation to date result from extreme events, seawalls and inappropriate development rather than sea-level rise alone. Here, we present the first analysis of coastal dynamics from a sea-level rise hotspot in the Solomon Islands. Using time series aerial and satellite imagery from 1947 to 2014 of 33 islands, along with historical insight from local knowledge, we have identified five vegetated reef islands that have vanished over this time period and a further six islands experiencing severe shoreline recession. Shoreline recession at two sites has destroyed villages that have existed since at least 1935, leading to community relocations. Rates of shoreline recession are substantially higher in areas exposed to high wave energy, indicating a synergistic interaction between sea-level rise and waves. Understanding these local factors that increase the susceptibility of islands to coastal erosion is critical to guide adaptation responses for these remote Pacific communities.

Methane and nitrous oxide emissions from a subtropical estuary(the Brisbane River estuary, Australia)

Methane (CH4) and nitrous oxide (N2O) are two key greenhouse gases. Their global atmospheric budgeting is, however, flout with challenges partly due to lack of adequate field studies determining the source strengths. Knowledge and data limitations exist for subtropical and tropical regions especially in the southern latitudes. Surface water methane and nitrous oxide concentrations were measured in a subtropical estuarine system in the southern latitudes in an extensive field study from 2010 to 2012 and water-air fluxes estimated using models considering the effects of both wind and flow induced turbulence. The estuary was found to be a strong net source of both CH4 and N2O all-year-round. Dissolved N2O concentrations ranged between 9.1 ± 0.4 to 45.3 ± 1.3 nM or 135 to 435% of atmospheric saturation level, while CH4 concentrations varied between 31.1 ± 3.7 to 578.4 ± 58.8 nM or 1210 to 26,430% of atmospheric saturation level. These results compare well with measurements from tropical estuarine systems. There was strong spatial variability with both CH4 and N2O concentrations increasing upstream the estuary. Strong temporal variability was also observed but there were no clear seasonal patterns. The degree of N2O saturation significantly increased with NOx concentrations (r(2)=0.55). The estimated water-air fluxes varied between 0.1 and 3.4 mg N2O m(-2)d(-1) and 0.3 to 27.9 mg CH4 m(-2)d(-1). Total emissions (CO2-e) were N2O (64%) dominated, highlighting the need for reduced nitrogen inputs into the estuary. Choice of the model(s) for estimation of the gas transfer velocity had a big bearing on the estimated total emissions.

Moreton Bay and its estuaries: a sub-tropical system under pressure from rapid population growth

Moreton Bay and its associated estuaries are an example of a complex aquatic system that is under increasing pressure from rapid population growth and urbanisation. Although the extent of decline in ecosystem health within Moreton Bay and its associated estuaries is significant and well documented, a range of innovative management responses have been implemented to reverse current declines. An overview of the development of Moreton Bay is provided, highlighting the dynamic and resilient nature of the system over geological time. The ecological responses that occur at decadal timeframes are presented along with a summary of the current state of the Bay’s ecology. The future challenges that are posed by predicted population increases, urbanisation and changes to the region’s climate are also discussed. The highly variable nature of the system over relatively short timeframes (i.e. flood vs non-flood conditions) as well as the ability of the system to adapt to long term changes (i.e. past morphological and ecosystem shifts) suggests that Moreton Bay and its associated estuaries have significant capacity to adapt to change. Whether the current rate of anthropogenically induced change is too rapid for the system to adapt, or whether such adaptions will be undesirable, is unable to be ascertained in any detail at this stage. Notwithstanding the above, the combination of a science-based management framework and the collaborative decision making processes that have been implemented to halt the decline of Moreton Bay have shown remarkable progress in a relatively short period of time.

Assessing the spatial and temporal variability of diffusive methane and nitrous oxide emissions from subtropical freshwater reservoirs.

Surface water-methane (CH4) and nitrous oxide (N2O) concentrations were measured and diffusive fluxes were estimated in three subtropical freshwater reservoirs (Little Nerang Dam (LND), Lake Wivenhoe (LW) and Lake Baroon (LB)) in southeast Queensland, Australia, during four seasons in 2011-2012. All reservoirs were strong sources of CH4 in all seasons. Surface water CH4 varied between 1350 and 524,000% saturation, and was overall highest in spring and summer, and lowest in winter, however, with no clear patterns common to all reservoirs. In contrast, all reservoirs switched from weak N2O sinks in spring to strong N2O sources for the rest of the year. N2O saturation in all reservoirs varied between 70 and 1230%. There were significant differences for CH4 concentrations and fluxes between the reservoirs. Within each reservoir, there was strong spatial CH4 variability but minimal N2O saturation variability. CH4 saturation was higher in inflow zones than in the main body. Area-weighted average fluxes were estimated using six water-air gas transfer velocity estimation models and resulted in fluxes in the range 4.8-20.5, 2.3-5.4, and 2.3-7.5 mg CH4 m(-2) d(-1), while N2O was 0.07-0.41, 0.09-0.22, and 0.03-0.09 mg N2O m(-2) d(-1) for LND, LW, and LB, respectively. Total emissions, in carbon dioxide equivalents, from all measurement campaigns were CH4 dominated (67-86%). The measured degree of CH4 saturation and fluxes are among the highest reported thus far indicating that subtropical freshwater reservoirs could be significant aquatic greenhouse gas sources. This paper provides a comprehensive assessment of the interplay between biogeochemical processes and the physical forcing driving the water-air gaseous emissions. The high variability coupled with the lack of consensus among estimation models calls for concerted efforts to address uncertainty of measurements for reliable emissions accounting.

Experimental evaluation of an Autonomous Surface Vehicle for water quality and greenhouse gas emission monitoring

This paper describes the experimental evaluation of a novel Autonomous Surface Vehicle capable of navigating complex inland water reservoirs and measuring a range of water quality properties and greenhouse gas emissions. The 16 ft long solar powered catamaran is capable of collecting water column profiles whilst in motion. It is also directly integrated with a reservoir scale floating sensor network to allow remote mission uploads, data download and adaptive sampling strategies. This paper describes the onboard vehicle navigation and control algorithms as well as obstacle avoidance strategies. Experimental results are shown demonstrating its ability to maintain track and avoid obstacles on a variety of large-scale missions and under differing weather conditions, as well as its ability to continuously collect various water quality parameters complimenting traditional manual monitoring campaigns.

Primary production of lake phytoplankton, dominated by the cyanobacterium Cylindrospermopsis raciborskii, in response to irradiance and temperature

Abstract We present the first data on the interacting effect of temperature and light on primary production of the toxic cyanobacterium Cylindrospermopsis raciborskii in situ. C. raciborskii can be a dominant component of the phytoplankton community in tropical and subtropical lakes and reservoirs. We examined the interacting effects of a range of light (0, 2, 7, 17, 30, and 100% of ambient light) and temperature (20, 24, 28, and 32 °C) conditions, in terms of primary production rate and primary production irradiance model parameters, for a C. raciborskii-dominated phytoplankton community in a subtropical reservoir. Based on 13C-uptake experiments, phytoplankton preconditioned to temperatures between 24 and 26 °C had highest maximum primary production rates (2.25 ± 0.45 μg C μg Chl-a−1 h−1) at 28 °C and lowest at 32 °C (0.58 ± 0.13 μg C μg Chl-a−1 h−1). Temperature also had an effect on the response to light conditions. Phytoplankton preconditioned to a shallow euphotic depth (~2.3 m deep) had the lowest half saturation of primary production, Ik, at 28 °C and highest at 32 °C, while the highest temperature treatment also had the highest level of photoinhibition at 100% of ambient light. This suggests that the cyanobacterial community is adapted to a low light environment under optimal temperature conditions for primary productivity. These conditions are consistent with other studies showing that C. raciborskii is highly adapted to low light conditions. This work demonstrates the importance of considering temperature when comparing calibrated primary production parameters.

Extreme rainfall and drinking water quality: a regional perspective

Record high, extreme rainfall intensities were observed in South East Queensland, Australia in January 2011, resulting in drinking water storages above capacity and subsequent floods. This paper focuses on changes, and subsequent recovery, in water quality due to sediment and nutrient loading associated with these high inflows across the regions water supplies. In total, 8 systems were routinely monitored and results indicate that storages with larger and more degraded catchment areas experienced greater declines in water quality relative to storages with smaller, less disturbed catchments. A case study focussing on Wivenhoe Dam, the primary water supply for the region, revealed large scale sediment inputs to the system, estimated to between 1.5 and 20 million tonnes. These were accompanied by high nutrient and metal loads, and water column turbidity above water quality guidelines persisted for a period of at least 6 months after inflows. Given the increased likelihood of such intense inflow events in the future, sound catchment management appears imperative to reduce negative impacts on storage water quality.

Nutrient exchange of extensive cyanobacterial mats in an arid subtropical wetland

Cyanobacterial mats cover extensive areas of subtropical arid coastal wetlands and are sites of active nutrient exchange. To assess spatial (low v. high in the intertidal zone) and temporal (day v. night) variability in nitrogen (N) exchange in arid Exmouth Gulf, Western Australia, we measured nutrient exchange (NOx–-N, NH4+ and soluble reactive phosphorus) during tidal inundation and N fixation of cyanobacterial mats before and during an unusual period of heavy rainfall. Additionally, we investigated the species composition within the cyanobacterial mat. We hypothesised that nutrients are released to the floodwater during tidal inundation, that N fixation is a significant path of N incorporation, that highest N fixation rates occur in the low intertidal zone at night, and that the cyanobacterial mat community composition varies across the intertidal zone. Our results showed that nutrients were removed from the floodwater during tidal inundation. N fixation accounted for 34% of N incorporation, with highest rates in the lower intertidal zone during the day. The cyanobacterial mat was dominated by Microcoleus chthonoplastes, but composition varied across the intertidal zone. The present study provided evidence of temporal and spatial variability in nutrient exchange and implied an important role of cyanobacterial mats in coastal production.

The impact of a high magnitude flood on metal pollution in a shallow subtropical estuarine embayment

Drought-breaking floods pose a risk to coastal water quality as sediments, nutrients, and pollutants stored within catchments during periods of low flow are mobilized and delivered to coastal waters within a short period of time. Here we use subtidal surface sediment surveys and sediment cores to explore the effects of the 2011 Brisbane River flood on trace metals zinc (Zn), lead (Pb), copper (Cu), nickel (Ni), chromium (Cr), manganese (Mn), and phosphorus (P) deposition in Moreton Bay, a shallow subtropical bay in eastern Australia. Concentrations of Zn, Cu, and Pb in sediments in central Moreton Bay derived from the 2011 flood were the highest yet observed in the Bay. We suggest flushing of metal rich sediments which had accumulated on the Brisbane River floodplain and in its estuary during the preceding 10 to 40years of low flows to be the primary source of this increase. This highlights the importance of intermittent high magnitude floods in tidally influenced rivers in controlling metal transport to coastal waters in subtropical regions.

Corals persisting in naturally turbid waters adjacent to a pristine catchment in Solomon Islands

Few water quality measurements exist from pristine environments, with fewer reported studies of coastal water quality from Solomon Islands. Water quality benchmarks for the Solomons have relied on data from other geographic regions, often from quite different higher latitude developed nations, with large land masses. We present the first data of inshore turbidity and sedimentation rate for a pristine catchment on Isabel Island. Surveys recorded relatively high coral cover. The lowest cover was recorded at 22.7% (Jejevo) despite this site having a mean turbidity (continuous monitoring) of 32 NTU. However, a similar site (Jihro) was significantly less turbid (2.1 mean NTU) over the same period. This difference in turbidity is likely due to natural features of the Jihro River promoting sedimentation before reaching coastal sites. We provide an important baseline for Solomon Island inshore systems, whilst demonstrating the importance of continuous monitoring to capture episodic high turbidity events.