Variation in stream organic matter processing among years and benthic habitats in response to forest clearfelling

We assessed rates of organic matter (OM) processing in coarse gravel and fine benthic sediment, along with water temperature, in four clearfell harvested and two undisturbed headwater streams flowing through wet eucalypt forest in southern Tasmania, Australia. Clearfell forestry in Tasmanian wet eucalypt forest involves felling of all timber followed by a high intensity regeneration burn to provide a receptive mineral seedbed for seedling growth. Bacterial carbon production and cellulose decomposition potential (together referred to as OM processing) were measured seasonally 3–5 years before and 2–4 years after harvesting in each stream. We employed a staircase design (staggered harvesting treatments) within a multiple before–after control–impact design to distinguish harvesting effects from natural variation. Clearfell harvesting raised the yearly mean water temperature by between 0.25 °C and 0.94 °C, and raised the maximum water temperature by between 0.84 and 1.6 °C. Rates of cellulose decomposition were not significantly correlated with sediment temperature but bacterial carbon production showed weak, significant correlations with temperature in fine (r = 0.20, P = 0.01, n = 137) and coarse gravel sediment (r = 0.39, P < 0.001, n = 137). The response in OM processing to clearfell harvesting differed between years and among benthic habitats. In coarse gravel habitat, there was a significant decrease in rates of cellulose decomposition potential in the 2nd and 4th year after harvesting, and a significant decrease in bacterial carbon production in the 3rd year after harvesting. However, we found a significant increase in rates of bacterial carbon production of fine sediment habitat in the 2nd and 4th year after harvesting. The contrasting response of OM processing between habitats indicates that habitat-specific changes occur after clearfell harvesting, which inhibit attempts to quantitatively predict downstream cumulative effects. Scaling up the habitat-specific responses will not only require estimates of the relative abundances of the distinct habitats, but may also require research into how different spatial configurations of habitats may affect reach- and catchment-scale estimates of OM processin

Fish assemblages as indicators of estuary ecosystem health

Understanding and managing increasing threat from diverse anthropogenic pressures on estuaries requires impact assessment and monitoring indices that provide accurate quantification of change and are readily communicable. Although indices based on nekton assemblage structure have obvious appeal to managers, the imperative to produce the most accurate measures possible has seen a move away from simple composite measures (such as diversity indices) towards complex multivariate approaches. However, complex methods often provide a poor basis for reporting because they can be difficult to report in terms that are meaningful to the end user. Effective indices should be simple to construct and communicate, relate directly to definable biological attributes, fall within predictable ranges for unimpacted systems and show demonstrable responses to known impacts. We use published nekton data for 30 natural and two artificial estuaries to develop a set of nekton assemblage-based summary measures that fit these criteria. We evaluated a suite of simple parallel measures based on both catch per unit effort (CPUE) and probability of encounter (PoE). Parallel measures provide complementary information thus a more robust assessments of change. Three measures fell within consistent bounds as long as comparisons were confined to the same time of year to remove the influence of seasonal variability, and were efficient at differentiating degraded from unimpacted estuaries. Because the successful approaches rely on PoE rather than CPUE they have considerable tactical advantages in that they are less destructive, allow for the collection of many more samples per unit time, and treat schooling and non-schooling species equivalently

Mixed responses of tropical Pacific fisheries and aquaculture to climate change

Pacific Island countries have an extraordinary dependence on fisheries and aquaculture. Maintaining the benefits from the sector is a difficult task, now made more complex by climate change. Here we report how changes to the atmosphere-ocean are likely to affect the food webs, habitats and stocks underpinning fisheries and aquaculture across the region. We found winners and losers - tuna are expected to be more abundant in the east and freshwater aquaculture and fisheries are likely to be more productive. Conversely, coral reef fisheries could decrease by 20% by 2050 and coastal aquaculture may be less efficient. We demonstrate how the economic and social implications can be addressed within the sector - tuna and freshwater aquaculture can help support growing populations as coral reefs, coastal fisheries and mariculture decline

Importance of freshwater flow in terrestrial–aquatic energetic connectivity in intermittently connected estuaries of tropical Australia

δ13C was used to identify seasonal variations in the importance of autochthonous and allochthonous sources of productivity for fish communities in intermittently connected estuarine areas of Australia’s dry tropics. A total of 224 fish from 38 species were collected from six intermittently connected estuarine pools, three in central Queensland (two dominated by C3 forest and one by C4 pasture) and three in north Queensland (one dominated by C3 and two by C4 vegetation). Samples were collected before and after the wet season. Fish collected in the two forested areas in central Queensland had the lowest δ13C, suggesting a greater incorporation of C3 terrestrial material. A seasonal variation in δ13C was also detected for these areas, with mean δ13C varying from −20 to −23‰ from the pre- to the post-wet season, indicating a greater incorporation of terrestrial carbon after the wet season. Negative seasonal shifts in fish δ13C were also present at the pasture site, suggesting a greater dependence on carbon of riparian vegetation (C3 Juncus sp.) in the post-wet season. In north Queensland, terrestrial carbon seemed to be incorporated by fish in the two C4 areas, as δ13C of most species shifted towards slightly heavier values in the post-wet season. A two-source, one-isotope mixing model also indicated a greater incorporation of carbon of terrestrial origin in the post-wet season. However, no seasonal differences in δ13C were detected for fish from the forested area of north Queensland. Overall, hydrologic connectivity seemed to be a key factor in regulating the ultimate sources of carbon in these areas. It is therefore important to preserve the surrounding habitats and to maintain the hydrologic regimes as close to natural conditions as possible, for the conservation of the ecological functioning of these areas