Predicting the seasonal evolution of southern African summer precipitation in the DePreSys3 prediction system

We assess the ability of the DePreSys3 prediction system to predict austral summer precipitation (DJF) over southern Africa, defined as the African continent south of 15°S. DePresys3 is a high resolution prediction system (at a horizontal resolution of ~ 60 km in the atmosphere in mid-latitudes and of the quarter degree in the Ocean) and spans the long period 1959–2016. We find skill in predicting interannual precipitation variability, relative to a long-term trend; the anomaly correlation skill score over southern Africa is greater than 0.45 for the first summer (i.e. lead month 2–4), and 0.37 over Mozambique, Zimbabwe and Zambia for the second summer (i.e. lead month 14–16). The skill is related to the successful prediction of the El-Nino Southern Oscillation (ENSO), and the successful simulation of ENSO teleconnections to southern Africa. However, overall skill is sensitive to the inclusion of strong La-Nina events and also appears to change with forecast epoch. For example, the skill in predicting precipitation over Mozambique is significantly larger for the first summer in the 1990–2016 period, compared to the 1959–1985 period. The difference in skill in predicting interannual precipitation variability over southern Africa in different epochs is consistent with a change in the strength of the observed teleconnections of ENSO. After 1990, and consistent with the increased skill, the observed impact of ENSO appears to strengthen over west Mozambique, in association with changes in ENSO related atmospheric convergence anomalies. However, these apparent changes in teleconnections are not captured by the ensemble-mean predictions using DePreSys3. The changes in the ENSO teleconnection are consistent with a warming over the Indian Ocean and modulation of ENSO properties between the different epochs, but may also be associated with unpredictable atmospheric variability

Resource Quantity Affects Benthic Microbial Community Structure and Growth Efficiency in a Temperate Intertidal Mudflat

Estuaries cover <1% of marine habitats, but the carbon dioxide (CO2) effluxes from these net heterotrophic systems contribute significantly to the global carbon cycle. Anthropogenic eutrophication of estuarine waterways increases the supply of labile substrates to the underlying sediments. How such changes affect the form and functioning of the resident microbial communities remains unclear. We employed a carbon-13 pulse-chase experiment to investigate how a temperate estuarine benthic microbial community at 6.5°C responded to additions of marine diatom-derived organic carbon equivalent to 4.16, 41.60 and 416.00 mmol C m−2. The quantities of carbon mineralized and incorporated into bacterial biomass both increased significantly, albeit differentially, with resource supply. This resulted in bacterial growth efficiency increasing from 0.40±0.02 to 0.55±0.04 as substrates became more available. The proportions of diatom-derived carbon incorporated into individual microbial membrane fatty acids also varied with resource supply. Future increases in labile organic substrate supply have the potential to increase both the proportion of organic carbon being retained within the benthic compartment of estuaries and also the absolute quantity of CO2 outgassing from these environments

Comparing benthic biogeochemistry at a sandy and a muddy site in the Celtic Sea using a model and observations

Results from a 1D setup of the European Regional Seas Ecosystem Model (ERSEM) biogeochemical model were compared with new observations collected under the UK Shelf Seas Biogeochemistry (SSB) programme to assess model performance and clarify elements of shelf-sea benthic biogeochemistry and carbon cycling. Observations from two contrasting sites (muddy and sandy) in the Celtic Sea in otherwise comparable hydrographic conditions were considered, with the focus on the benthic system. A standard model parameterisation with site-specific light and nutrient adjustments was used, along with modifications to the within-seabed diffusivity to accommodate the modelling of permeable (sandy) sediments. Differences between modelled and observed quantities of organic carbon in the bed were interpreted to suggest that a large part (>90%) of the observed benthic organic carbon is biologically relatively inactive. Evidence on the rate at which this inactive fraction is produced will constitute important information to quantify offshore carbon sequestration. Total oxygen uptake and oxic layer depths were within the range of the measured values. Modelled depth average pore water concentrations of ammonium, phosphate and silicate were typically 5–20% of observed values at the muddy site due to an underestimate of concentrations associated with the deeper sediment layers. Model agreement for these nutrients was better at the sandy site, which had lower pore water concentrations, especially deeper in the sediment. Comparison of pore water nitrate with observations had added uncertainty, as the results from process studies at the sites indicated the dominance of the anammox pathway for nitrogen removal; a pathway that is not included in the model. Macrofaunal biomasses were overestimated, although a model run with increased macrofaunal background mortality rates decreased macrofaunal biomass and improved agreement with observations. The decrease in macrofaunal biomass was compensated by an increase in meiofaunal biomass such that total oxygen demand remained within the observed range. The permeable sediment modification reproduced some of the observed behaviour of oxygen penetration depth at the sandy site. It is suggested that future development in ERSEM benthic modelling should focus on: (1) mixing and degradation rates of benthic organic matter, (2) validation of benthic faunal biomass against large scale spatial datasets, (3) incorporation of anammox in the benthic nitrogen cycle, and (4) further developments to represent permeable sediment processes

Carbon and nitrogen cycling on intertidal mudflats in a temperate Australian estuary

The sources of organic matter, benthic metabolism (light/dark 02 and TCO2 fluxes), benthic dissolved nitrogen fluxes, denitrification, nitrogen fixation and sediment NH4+ production were studied on the upper and lower regions of two mudflats in Huon Estuary, south east Tasmania over four seasons. One study site was located in the upper euryhaline part of the estuary and the other study site was located in a \marine\" side arm of the estuary. The aim of the study was to develop a detailed conceptual understanding of sedimentary nitrogen cycling processes in relation to the activity of microphytobenthos (MPB) in this system. The organic matter pool at both sites was generally dominated by that derived from terrestrial sources. Organic matter derived from microphytobenthos generally only comprised a small fraction of the organic matter pool. Compound-specific stable isotope ratio analysis of bacterial and algal fatty acids suggested the algal-derived fraction of organic matter was most likely the driver of bacterial respiration within the sediment. As such this fraction of organic matter had a high turnover rate and never built up to significant amounts. The MPB at both sites consisted of a mixed community of diatoms chlorophytes and cyanobacteria the relative composition of which varied with site position on the mudflat and season. Rates of primary production by MPB were influenced by an exposure to wave energy and an availability of light. Rates of primary production by MPB were significantly greater on the upper mudflat than the lower mudflat at the site in the upper estuary. It is proposed this arose as a consequence of light limitation across the inundation gradient caused by high concentrations of coloured dissolved organic matter in the water at this site. Benthic respiration at this site was controlled by temperature as well as organic matter input from MPB. At the site in the marine side arm of the estuary rates of primary production were not significant different between the upper and lower mudflat and were significantly lower than at the site in the upper estuary. A greater exposure to wave energy as indicated by sediment grain size and aspect was the most likely cause of the lower rates of primary production at this site. As a consequence both the upper and lower mudflats at the site in the upper estuary were autotrophic on an annual basis while both the upper and lower mudflats at the site in the marine side arm of the estuary were heterotrophic on an annual basis. The balance between production and respiration was of fundamental importance in determining whether the sediments were a net source or sink for dissolved inorganic nitrogen with autotrophic sediments showing a net uptake of nitrogen and heterotrophic sediments showing a net release. Primary production also influenced the exchange of gaseous nitrogen species. Rates of denitrification were generally very low and negatively correlated with rates of primary production while rates of N2 fixation were at times high and were positively correlated with primary production. Dissolved nitrogen fluxes were dominated by dissolved organic nitrogen (DON) where and when high rates of production (uptake of DON) and respiration (release of DON) were observed. MPB also profoundly influenced the nitrogen cycle through the production of labile but high C:N ratio organic material. At times of high primary production the calculated demand for nitrogen - based on simple but widely used stoichiometric models - was found to be well in excess of the measured uptake. Subsequent measurements of N2 fixation using the acetylene reduction assay (calibrated using 15N-N2) showed that N2 fixation could at times account for the observed deficit in nitrogen uptake. In general however N2 fixation could not account for the deficit in dissolved nitrogen assimilation. It is suggested that a stoichiometric relationship between carbon and nitrogen assimilation reflecting the C:N ratio of algal cells will only occur during the initial development of the MPB biofilm. Once the MPB biofilm has become established the majority of carbon assimilation is directed into the production of extracellular organic carbon (EOC) such as extracellular polymeric substances (EPS) rather than cell growth. It is proposed that the input of this labile but high C:N ratio organic material to the sediment drove bacterial respiration as well as stimulating bacterial nitrogen reassimilation. As a consequence the ratio of TCO2:NI-14+produced within the sediment was generally in excess of 15 and in some cases in excess of 60.

Effects of varying levels of nutrient inputs to coastal marine systems: a case study of a semi-enclosed bay influenced by a large urban population

Port Phillip Bay (PPB) is the largest marine bay on the Australian coast and is the site of Australia’s second largest city, Melbourne. A major environmental study in the 1990s recommended a reduction in the nitrogen (N) input to the bay. Subsequently, improvements to sewage treatment efficiency in the 2000s coincided with the longest and most severe drought in recorded history, resulting in N inputs dropping by more than half in the 2000s compared with the 1990s. Here we review studies conducted over the past 30 years to understand the effects of varying nutrient levels on the ecology of PPB. Studies showed that PPB is an N limited system both in time and space. Biological productivity in PPB was markedly affected by reduced N loads during the drought, resulting in declines in seagrass and kelp cover, as well as benthic fish biomass. Overall, research has shown that while setting conservative limits on N input will effectively negate the risk of widespread eutrophication, there will likely be a tradeoff in reduced bay productivity. Managers will need to consider both sides of this equation when managing the load of N entering PPB, and potentially other marine systems around the world