Craig Woodward

The hydrological legacy of deforestation on global wetlands

Increased catchment erosion and nutrient loading are commonly recognized impacts of deforestation on global wetlands. In contrast, an increase in water availability in deforested catchments is well known in modern studies but is rarely considered when evaluating past human impacts. We used a Budyko water balance approach, a meta-analysis of global wetland response to deforestation, and paleoecological studies from Australasia to explore this issue. After complete deforestation, we demonstrated that water available to wetlands increases by up to 15% of annual precipitation. This can convert ephemeral swamps to permanent lakes or even create new wetlands. This effect is globally significant, with 9 to 12% of wetlands affected, including 20 to 40% of Ramsar wetlands, but is widely unrecognized because human impact studies rarely test for it. Ancient Australasian deforestations created wetlands where none previously existed. Impacts of deforestation on wetlands Deforestation worldwide may be causing an increase in the extent of wetlands. Using a combination of different approaches, Woodward et al. show that ancient and more recent deforestation has resulted in major changes in global wetland hydrology. For example, in Australia and New Zealand, deforestation has created new wetlands or increased the water level in existing wetlands. Recognition of this effect has implications for landscape management: Reforestation programs in wetland catchments may have unintended consequences for vulnerable wetlands. Science, this issue p. 844

Carbon and nitrogen stable isotope ratios in surface sediments from lakes of western Ireland: implications for inferring past lake productivity and nitrogen loading

We used statistical analyses to determine which subset of 36 environmental variables best explained variations in surface sediment δ13C and δ15N from 50 lakes in western Ireland that span a human-impact gradient. The factors controlling lake sediment δ13C and δ15N depended on whether organics in the lake sediment were mostly derived from the lake catchment (allochthonous) or from productivity within the lake (autochthonous). Lake sediments with a dominantly allochthonous organic source (high C:N ratio sediments) produced δ13C and δ15N measurements similar to values from catchment vegetation. δ13C and δ15N measurements from lake sediments with a dominantly autochthonous organic source (low C:N ratio sediments) were influenced by fractionation in the lake and catchment leading up to assimilation of carbon and nitrogen by lacustrine biota. δ13C values from lake sediment samples in agricultural catchments were more negative than δ13C values from lake sediment samples in non-impacted, bogland catchments. Hypolimnetic oxygen concentrations and methane production had a greater influence on δ13C values than fractionation due to algal productivity. δ15N from lake sediment samples in agricultural catchments were more positive than δ15N in non-impacted bogland catchments. Lower δ15N values from non-impacted lake catchments reflected δ15N values of catchment vegetation, while higher δ15N values in agricultural catchments reflected the high δ15N values of cattle manure and inorganic fertilisers. The influence of changing nitrogen sources and lake/catchment fractionation processes were more important than early diagenesis for lake sediment δ15N values in this dataset. The results from this study suggest a possible influence of bound inorganic nitrogen on the bulk sediment δ15N values. We recommend using a suitable method to control for bound inorganic nitrogen in lake sediments, especially when working with clay-rich sediments. This study confirms the usefulness of δ13C and δ15N from bulk lake sediments, as long as we are mindful of the multiple factors that can influence these values. This study also highlights how stable isotope datasets from lake surface sediments can complement site-specific isotope source/process studies and help identify key processes controlling lake sediment δ13C and δ15N in a study area.

A Holocene record of climate and hydrological changes from Little Llangothlin Lagoon, south eastern Australia

We present a new well dated Holocene record of environmental change from Little Llangothlin Lagoon in eastern Australia derived from aquatic plant macrofossils, macroscopic charcoal flux, and sediment stratigraphy from multiple cores. Little Llangothlin was an ephemeral freshwater wetland exhibiting frequent dry phases between 9800 and 9300 calendar years before present (cal. yr BP). There was a switch to a more positive water balance after 9300 cal. yr BP, and by 8000 cal. yr BP, there was a lake that persisted until 6100 cal. yr BP. The period between 6100 and 1000 cal. yr BP was much drier, and there is no evidence for a permanent lake during this period. The Little Llangothlin record provides evidence for a wet phase during the Early to Middle Holocene (9000–6000 cal. yr BP) from the boundary region between temperate and tropical influences in eastern Australia. We propose that generally enhanced circulation after 9000 cal. yr BP explains the pattern of increasing moisture at the site at this time. The later Holocene climate at the site is consistent with other sites in south east Australia with a switch to generally drier conditions after 6000 cal. yr BP.

A snapshot of the limnology of eastern Australian water-bodies spanning the tropics to Tasmania: the land-use, climate, limnology nexus

This is a study of 45 natural and artificial water bodies extending across the whole of eastern Australia from the tropics to Tasmania. A broad variety of physio-chemical, land-use and climatic parameters were measured. Reservoirs and other artificial water bodies respond to stressors in their catchments in a similar fashion to natural lakes but tend to be less nutrient rich possibly due to shorter residence times and active management. pH and salinity are strongly correlated in the dataset. Bedrock has a strong influence on pH in freshwater lakes but all highly saline lakes are alkaline, irrespective of bedrock. High concentrations of anions in saline lakes preclude the existence of acid conditions by binding available hydrogen ions. Almost all lakes fall on salinity axes that indicate marine origin for their salts. An assessment of the TN:TP molar ratios from the lakes in this dataset indicates that productivity in Australian lakes could be limited by both nitrogen and phosphorus. Future research using macro-nutrient enrichment experiments should be pursued to confirm this preliminary observation. There is a strong positive correlation between regional aridity and lake eutrophication. This is typical of semi-arid and seasonally arid environments and reflects the concentration of nutrients due to evaporative flux in shallow basins with high residence times.

Major disturbance to aquatic ecosystems in the South Island, New Zealand, following human settlement in the Late Holocene

Lake sediment records from three lakes in the South Island of New Zealand were examined to determine the effects of human (Māori and European) impacts on the lake catchments during the Late Holocene. Major changes in lake biota occurred in the Early to Middle Holocene (11,000–6000 cal. yr BP), but there were no major changes between c. 6000 cal. yr BP and the time of human impact. Intensive Māori forest clearance occurred here between c. ad 1200 and 1600, which is consistent with other New Zealand records. Catchment erosion and increased sedimentation probably occurred in all of the studied lakes, but the most obvious changes occurred in Lake Clearwater and the Māori Lakes. There was evidence for gravity-induced slumping of the littoral sediments in Lake Clearwater due to increased sediment loading, and the outflow from the Māori Lakes was blocked by a migrating alluvial fan. The erosion of sediment (and nutrients) from the lake catchments led to eutrophication, but increases in lake depth were just as important in two of the lakes. Increased water depth was caused by damming of the Māori lakes outflow by a migrating alluvial fan. Reduced evapotranspiration following deforestation would also have led to increased water yield in lake catchments. European impacts were minor compared with the impacts of Māori deforestation, and all lakes display different levels of recovery towards pre-human impact conditions. Complete recovery is prevented by permanent changes in catchment hydrology and probable internal feedback mechanism such as wind-induced sediment re-suspension in the larger lakes.

Eutrophication of a small, deep lake in southern New Zealand: the effects of twentieth-century forest clearance, changing nutrient influx, light penetration and bird behaviour

This study provides a high resolution multi-proxy record of the response of an aquatic ecosystem (Alexander Lake) to forest clearance in New Zealand in the late twentieth century (ca. 1950–2006 AD). New chironomid-based transfer functions for lake water total nitrogen (TN) concentration were applied to the Alexander Lake chironomid record. A test of the significance of reconstructions based on multiple model types indicates that a model with the highest r2 and lowest root mean squared error of prediction may not necessarily perform the best when applied to a particular site. The chironomid-based TN reconstruction and other proxies suggest a complex response by a stained water (dystrophic) lake in a forested catchment to deforestation. Minor perturbations and nutrient influx may favour increased phytoplankton production, but continued light attenuation by dissolved organic carbon and humic compounds prevents proliferation of submerged macrophytes. Complete mechanical forest clearance resulted in a short term pulse of nutrients and eutrophication. The long term effect of deforestation was to increase light penetration and favour the growth of submerged macrophytes. Continued eutrophication of Alexander Lake could be due to a contribution of bird-derived nutrients. Deforestation around Alexander Lake has created a perfect moulting site for Paradise Shelducks (Tadorna variegata Gmelin). The input of total phosphorus from T. variegata could be enough to trigger blooms of Microcystis that currently occur in the lake. Changes in bird behaviour in response to changes in vegetation should therefore be considered a possible result of past (including prehistoric) and future deforestation in New Zealand.

Coring and augering

This chapter introduces the principles of coring, including the objectives for taking a good core and the major factors that should be considered to ensure the collection of nondisturbed, representative core samples. The chapter also provides an overview of the design, function, and operation of the main types of coring equipment that can be used to sample sediments from a variety of settings including lakes, the ocean, peat bogs, soils, and permafrost. The major advantages and disadvantages of each type of corer are also discussed.

Holocene ecosystem change in Little Llangothlin Lagoon, Australia: implications for the management of a Ramsar-listed wetland

We present new chironomid and stable isotope (δ13C and δ15N) data from Little Llangothlin Lagoon, Australia that provides more detail on changes in this wetland since European settlement ca. 1840 AD. We also examine how the updated Holocene paleoecological record provides insights for management of this Ramsar-listed wetland. The current management strategy for Little Llangothlin is to restore the wetland and catchment to its natural state. This strategy is intended to protect the values that allowed it to be listed as a Ramsar wetland; i.e. its role as a drought refuge for waterbirds and to preserve or enhance threatened ecological communities. There are clear conflicts between the Ramsar listing criteria, management objectives and the management strategy in light of information provided by the palaeoecological record. In particular, restoration of terrestrial ecosystems through reforestation may jeopardise the wetlands role as a drought refuge. Some activities, such as artificial raising of the water level in 1989 are intended to restore, but actually introduced a state that did not exist prior to human settlement. We recommend a more integrated management approach that heeds the information provided by the palaeoecological record and focuses more on maintenance or enhancement of ecosystem services and biodiversity.