Groundwater estuaries of salt lakes: buried pools of endemic biodiversity on the western plateau, Australia

Subterranean or groundwater estuaries occur in porous and cavernous substrates where groundwater abuts the ocean. Like surface estuaries, they are strongly stratified, temporally and hydrochemically heterogeneous environments that support complex hydrogeochemical and biological processes and ecological communities. Here, we contend that groundwater estuaries also occur where groundwater flow approaches salt lakes and provide evidence in the context of groundwater (valley or phreatic) calcretes in palaeovalleys of the arid western plateau of Australia. The calcrete groundwater estuaries display marked and complex physico-chemical gradients along, across and through the groundwater flow path. From the first principles and the density differences between water bodies, we may expect the form and dynamics of the saltwater front to mimic that of marine estuaries but with the dynamic and temporal response to changing hydrology heavily dampened, and driven by the episodic groundwater recharge and lake filling typical of arid regions. The calcrete aquifers support diverse biological communities of obligate groundwater animals, largely endemic to a given calcrete body. These communities comprise both macro and microinvertebrates, predominantly a suite of crustacean higher taxa, and a great diversity of diving beetles (Dytiscidae) isolated in the calcrete aquifers between ca. 5 and 8 million years ago. Guest Editors: J. John & B. Timms Salt Lake Research: Biodiversity and Conservation—Selected papers from the 9th Conference of the International Society for Salt Lake Research An erratum to this article can be found at     

Aquatic ecosystem responses to fire and flood size in the Okavango Delta: observations from the seasonal floodplains

The frequency of fires in the Okavango Delta seasonal floodplains peaked at an intermediate frequency of flooding. Floodplains are commonly burnt every 3–5 years. This study showed fundamental changes in ecosystem properties due to burning. A burnt seasonal floodplain in the aquatic phase had oxygen levels well above saturation, 100–200%, while the levels in the un-burnt control site were below saturation and, at night, could decline to 10–40% saturation. The total phosphorous and total nitrogen concentrations were similar on both floodplains but considerably enriched relative to inflowing water, due to nutrient release from the flooded soil-sediment and animal droppings. Zooplankton biomass was very high in both systems although the abundance of fish fry was ten times higher on the un-burnt floodplain. In a low flood year the un-burnt floodplain water had high nutrient levels, primary production, methane emission, and subsequent uptake of methane in biota, as well as a high zooplankton biomass. The very high flood the following year showed the opposite with much lower production at all levels owing primarily to greater dilution of nutrients. The abundance of fish, however, was much higher during the high flood year. Macrophytes and litter provide direct shelter for fish fry but also promote low oxygen levels when decaying. Large flooded areas result in high fish production by removing obstacles related to congestion. This interplay between hydroperiod and fire may be crucial for the maintenance of high biological productivity both in the aquatic and terrestrial phases in a very nutrient poor wetland landscape. Understanding these interactions is crucial for optimal management.     

Biogeochemical Contributions of Tree Islands to Everglades Wetland Landscape Nitrogen Cycling During Seasonal Inundation

In the Florida Everglades, tree islands are conspicuous heterogeneous elements in the herbaceous wetland landscape. We characterized the biogeochemical role of a seasonally flooded tree island during wet season inundation, specifically examining hydrologically mediated flows of nitrogen (N) and N retention by the tree island. We estimated ecosystem N standing stocks and fluxes, soil and litter N transformation rates, and hydrologic fluxes of N to quantify the net ecosystem N mass flux. Results showed that hydrologic sources of N were dominated by surface water loads of nitrate (NO₃) and ammonium (NH₄). Nitrate immobilization by soils and surficial leaf litter was an important sink for surface water dissolved inorganic N (DIN). We estimated that the net annual DIN retention by a seasonally flooded tree island was 20.5 ± 5.0 g m⁻² during wet season inundation. Based on the estimated tree island surface water DIN loading rate, a seasonally flooded tree island retained 76% of imported DIN. As such, seasonally flooded tree islands have the potential to retain 55% of DIN entering the marsh landscape via upstream canal overland flow in the wet season. By increasing reactive surface area and DOC availability, we suggest that tree islands promote convergence of elements that enhance DIN retention. Tree islands of this region are thus important components of landscape-scale restoration efforts that seek to reduce sources of anthropogenic DIN to downstream estuaries.     

The effects of tree establishment on water and salt dynamics in naturally salt-affected grasslands

Plants, by influencing water fluxes across the ecosystem–vadose zone–aquifer continuum, can leave an imprint on salt accumulation and distribution patterns. We explored how the conversion of native grasslands to oak plantations affected the abundance and distribution of salts on soils and groundwater through changes in the water balance in naturally salt-affected landscapes of Hortobagy (Hungary), a region where artificial drainage performed ∼150 years ago lowered the water table (from −2 to −5 m) decoupling it from the surface ecosystem. Paired soil sampling and detailed soil conductivity transects revealed consistently different salt distribution patterns between grasslands and plantations, with shallow salinity losses and deep salinity gains accompanying tree establishment. Salts accumulated in the upper soil layers during pre-drainage times have remained in drained grasslands but have been flushed away under tree plantations (65 and 83% loss of chloride and sodium, respectively, in the 0 to −0.5 m depth range) as a result of a five- to 25-fold increase in infiltration rates detected under plantations. At greater depth, closer to the current water table level, the salt balance was reversed, with tree plantations gaining 2.5 kg sodium chloride m⁻² down to 6 m depth, resulting from groundwater uptake and salt exclusion by tree roots in the capillary fringe. Diurnal water table fluctuations, detected in a plantation stand but not in the neighbouring grasslands, together with salt mass balances suggest that trees consumed ∼380 mm groundwater per year, re-establishing the discharge regime and leading to higher salt accumulation rates than those interrupted by regional drainage practices more than a century ago. The strong influences of vegetation changes on water dynamics can have cascading consequences on salt accumulation and distribution, and a broad ecohydrological perspective that explicitly considers vegetation–groundwater links is needed to anticipate and manage them.     

Erosion, Geological History, and Indigenous Agriculture: A Tale of Two Valleys

Irrigated pondfields and rainfed field systems represented alternative pathways of agricultural intensification that were unevenly distributed across the Hawaiian Archipelago prior to European contact, with pondfields on wetter soils and older islands and rainfed systems on fertile, moderate-rainfall upland sites on younger islands. The spatial separation of these systems is thought to have contributed to the dynamics of social and political organization in pre-contact Hawai’i. However, deep stream valleys on older Hawaiian Islands often retain the remains of rainfed dryland agriculture on their lower slopes. We evaluated why rainfed agriculture developed on valley slopes on older but not younger islands by comparing soils of Pololū Valley on the young island of Hawai’i with those of Hālawa Valley on the older island of Moloka’i. Alluvial valley-bottom and colluvial slope soils of both valleys are enriched 4–5-fold in base saturation and in P that can be weathered, and greater than 10-fold in resin-extractable P and weatherable Ca, compared to soils of their surrounding uplands. However, due to an interaction of volcanically driven subsidence of the young island of Hawai’i with post-glacial sea level rise, the side walls of Pololū Valley plunge directly into a flat valley floor, whereas the alluvial floor of Hālawa Valley is surrounded by a band of fertile colluvial soils where rainfed agricultural features were concentrated. Only 5% of Pololū Valley supports colluvial soils with slopes between 5° and 12° (suitable for rainfed agriculture), whereas 16% of Hālawa Valley does so. The potential for integrated pondfield/rainfed valley systems of the older Hawaiian Islands increased their advantage in productivity and sustainability over the predominantly rainfed systems of the younger islands.     

Response of riparian plant communities to distance from surface water in the Okavango Delta,Botswana

The objective of this study was to determine the influence of distance from surface water on riparian woodland communities in the Okavango Delta. Vegetation sampling was conducted in seven sites within the Okavango Delta in 20 m × 10 m belted plots placed perpendicular to the river bank. The plots were placed at 0–10 m, 10–20 m, 20–30 m, 30–40 m and 40–50 m distance classes increasing away from the river bank. Tree height, basal area, species richness, canopy cover and diversity were determined for each distance class. Indicator species analysis was used to determine the characteristic species at each distance class. Single‐factor ANOVA and Tukey post hoc analysis were used to compare species diversity, mean tree height, cover and basal area between distance classes. Correlation between distance from surface water and vegetation parameters was sought using Spearman regression analysis. All parameters except for species richness varied significantly (P < 0.05) along distance from surface water. Distance from surface water was positively correlated all vegetation parameters except for mean species richness/plot. These results show that distance from surface water influences riparian plant community composition and distribution in the Okavango Delta. This implies that riparian plant species can be indicators of long‐term hydrologic conditions in the Delta.     

Seasonal dynamics of macrohabitat use by small mammals in the Okavango Delta,Botswana: implications for landscape‐level disturbance resilience

In a changing climate, it is imperative to understand potential ecosystem resilience at all taxonomic levels. I compare seasonal small mammal utilization of woodlands (tree islands) and grasslands in the Okavango Delta, Botswana, to elucidate macrohabitat relationships and to test whether the two macrohabitats are similar in their ability to serve as a source of colonizers for the other after disturbance. Capture–mark–recapture data revealed that abundances for Dendromus mesomelas and Gerbilliscus leucogaster were higher in grasslands than tree islands, while Mus indutus abundance appeared higher in the grasslands in the dry season but roughly equal in the rainy season. Mastomys spp. and Steatomys pratensis maintained low levels in the grassland habitat throughout the year and experienced a population peak in the tree island habitat during the rainy season. There were no significant differences in sex ratio, mean mass or breeding condition. Dominance and total biomass were higher in the grasslands with the trend more pronounced in the rainy season. Terrestrial small mammals in the Okavango Delta employ differing strategies in macrohabitat selection and some exploit tree islands when herbaceous cover is present. Metacommunity dynamics exist for some species, and both habitats can serve as a source of colonizers under certain conditions.     

Potential environmental impacts associated with the proposed abstraction of water from the Okavango River in Namibia

The Namibian Department of Water Affairs has in the past faced considerable pressure to relieve the water shortages caused by recent droughts. One of the options considered in 1996, following poor runoff during the 1994/95 and 1995/96 seasons, was a proposal to abstract some 17Mm³ of water per year from the Okavango River at Rundu, and transfer this via a 260km long pipeline to the head of the Eastern National Water Carrier at the town of Grootfontein. Part of the overall evaluation of this scheme included an assessment of the potential environmental impacts that could arise. An initial environmental evaluation was conducted from a point approximately 40 kilometres upstream of Rundu in Namibia, to the distal end of the Okavango Delta at Maun in Botswana.

Hydrological studies showed that the proposed abstraction represented a reduction of approximately 0.32% in the mean annual flow of the Okavango River at Rundu. The abstraction represents 0.17% of the mean annual flow at Mukwe, downstream of the Cuito River confluence. The adverse effects of the proposed water abstraction scheme would be extremely small along the Okavango River in Namibia, whilst outflows from the lower end of the Okavango Delta to the Thamalakane River would be reduced by some 1.44Mm³/year (11%). Additional studies showed that these effects could be reduced by some 10–13% if abstraction was confined to the falling limb of the hydrograph.

Hydrological simulations have shown that the maximum likely loss of inundated area in the Okavango Delta would total approximately 7km² out of some 8 000km². This potential loss in inundated area would be concentrated in the lower reaches of the seasonal swamps and seasonally inundated grasslands, specifically in the lower reaches of the Boro, Gomoti, Santantadibe and Thaoge channels. However, these effects would most likely be expressed as a shoreline effect, with the loss in area spread out over the shoreline and periphery of islands and would not be restricted to a single specific area. This anticipated loss in inundated area is unlikely to have measurable impacts on environmental components.

Overall, the study found no 'fatal flaws' which would prevent the water abstraction scheme from proceeding and the anticipated effects on the Okavango system are more likely to be seen in the Okavango Delta, rather than along the Okavango River. The anticipated ecological implications of the scheme were small in spatial extent and are unlikely to be perceptible against the natural year-to-year variability in inundation of the Okavango Delta or outflows to the Thamalakane River. However, the public perceptions of the proposed water transfer project were strongly negative and appeared to be at least in part due to the very low water levels in the Okavango River and Okavango Delta during the past three years and during the study period. These low water levels were amongst the lowest on record and it is likely that the public would attribute any adverse effect recorded in the future to the abstraction scheme, whether this were true or not. These negative perceptions of the desirability and acceptability of the proposed scheme were strongly linked to potential adverse affects on the tourism industry along the Okavango River and in the Okavango Delta, with possible adverse economic effects on local residents.     

Effect of Watering and Soil Moisture on Mercury Emissions from Soils

This paper presents data from experiments that measured Mercury (Hg) flux as a function of water addition and subsequent soil drying, and maintenance of soil water content over time utilizing small dynamic gas exchange chambers and large mesocosms. When soil surfaces were dry and water was added at an amount less than that necessary to saturate the soil an immediate large (relative to dry soil flux) release of Hg occurred. Diel Hg emissions from soils, unenriched (0.02 μg g⁻¹) and enriched (3 μg g⁻¹) in Hg and wet below saturation, were significantly elevated above that occurring from dry soils (2–5 times depending on soil water content) for weeks to months. Enhancement of emissions from wet soils in direct sunlight were greater than that from soils shaded or in the dark suggesting that a synergism exists between soil moisture and light. When soils were watered to saturation Hg emissions were suppressed or remained the same depending on the degree of saturation. It is hypothesized that the addition of soil water in amounts less than that necessary to saturate the soil surface results in an immediate release of elemental Hg from soil surface as the more polar water molecule out competes Hg for binding sites. As the water moves into the soil, Hg adsorbed to soil particles is desorbed into soil gas and dissolved in the soil water. The process of evaporation facilitates movement of Hg as mass flow to the soil surface where it is made available for subsequent release. The latter is hypothesized to be an important process by which Hg is recharged at the soil–air interface.     

Water and Nutrient Dynamics in Surface Roots and Soils are not Modified by Short-term Flooding of Phreatophytic Plants in a Hyperarid Desert

Little is known of the mechanisms employed by woody plants to acquire key resources such as water and nutrients in hyperarid environments. For phreatophytic plants, deep roots are necessary to access the water table, but given that most nutrients in many desert ecosystems are stored in the upper soil layers, viable shallow roots may be equally necessary for nutrient uptake. We sought to better understand the interaction between water and nutrient uptake from soil horizons differing in the relative abundance of these resources. To this end, we monitored plant water and nutrient status before and after applying flood irrigation to four phreatophytic perennial plant species in the remote hyperarid Taklamakan desert in western China. Sap flow in the roots of five plants of the perennial desert species Alhagi sparsifolia Shap., Karelina caspica (Pall.) Less., Calligonum caput medusea Schrenk, and Eleagnus angustifolia Hill. was monitored using the heat ratio method (HRM). Additionally we measured predawn and midday water potential, foliar nitrate reductase activity (NRA), xylem sap nutrient concentration and the concentration of total solutes in the leaves before, 12 and 96 h after flooding to investigate possible short-term physiological effects on water and nutrient status. Rates of sap flow measured during the day and at night in the absence of transpiration did not change after flooding. Moderately high rates of sap flow (HRM heat pulse velocity, 5–25 cm h⁻¹) detected during the day in soils that had a near zero water content at the surface indicated that all species had contact to groundwater. There was no evidence from sap flow data that plants had utilised flood water to increase maximum rates of transpiration under similar climatic conditions, and there was no evidence of a process to improve the efficiency of water or nutrient uptake, such as hydraulic redistribution (i.e. the passive movement of water from moist soil to very dry soil via roots). Measurements of plant water status, xylem sap nutrient status, foliar NRA and the concentration of osmotically active substances were also unaffected by flood irrigation. Our results clearly show that groundwater acts as the major source of water and nutrients for these plants. The inability of plants to utilise abundant surface soil–water or newly available nutrients following irrigation was attributed to the absence of fine roots in the topsoil layer.     

Denitrification in the vadose zone and in surficial groundwater of a sandy entisol with citrus production

A portion of nitrate (NO ₃ ⁻ ), a final breakdown product of nitrogen (N) fertilizers, applied to soils and/or that produced upon decomposition of organic residues in soils may leach into groundwater. Nitrate levels in water excess of 10 mg L⁻¹ (NO₃–N) are undesirable as per drinking water quality standards. Nitrate concentrations in surficial groundwater can vary substantially within an area of citrus grove which receives uniform N rate and irrigation management practice. Therefore, differences in localized conditions which can contribute to variations in gaseous loss of NO ₃ ⁻ in the vadose zone and in the surficial aquifer can affect differential concentrations of NO₃–N in the groundwater at different points of sampling. The denitrification capacity and potential in a shallow vadose zone soil and in surficial groundwater were studied in two large blocks of a citrus grove of ‘Valencia’ orange trees (Citrus sinensis (L.) Obs.) on Rough lemon rootstock ( Citrus jambhiri (L.)) under a uniform N rate and irrigation program. The NO₃–N concentration in the surficial groundwater sampled from four monitoring wells (MW) within each block varied from 5.5- to 6.6-fold. Soil samples were collected from 0 to 30, 30 to 90, or 90 to 150 cm depths, and from the soil/groundwater interface (SGWI). Groundwater samples from the monitoring wells (MW) were collected prior to purging (stagnant water) and after purging five well volumes. Without the addition of either C or N, the denitrification capacity ranged from 0.5 to 1.53, and from 0.0 to 2.25 mg N₂O–N kg⁻¹ soil at the surface soil and at the soil/groundwater interface, respectively. The denitrification potential increased by 100-fold with the addition of 200 mg kg⁻¹ each of N and C. The denitrification potential in the groundwater also followed a pattern similar to that for the soil samples. Denitrification potential in the soil or in the groundwater was greatest near the monitor well with shallow depth of vadose zone (MW3). Cumulative N₂O–N emission (denitrification capacity) from the SGWI soil samples and from stagnant water samples strongly correlated to microbial most probable number (MPN) counts (r² = 0.84 – 0.89), and dissolved organic C (DOC) (r² = 0.96 – 0.97). Denitrification capacity of the SGWI samples moderately correlated to water-filled pore space (WFPS) (r² = 0.52). However, extractable NO₃-N content of the SGWI soil samples poorly (negative) correlated to denitrification capacity (r² = 0.35). However, addition C, N or both to the soil or water samples resulted in significant increase in cumulative N₂O emission. This study demonstrated that variation in denitrification capacity, as a result of differences in denitrifier population, and the amount of readily available carbon source significantly (at 95% probability level) influenced the variation in NO₃–N concentrations in the surficial groundwater samples collected from different monitoring wells within an area with uniform N management. This revised version was published online in June 2006 with corrections to the Cover Date.     

Small off-shore islands can serve as important refuges for endemic beetle conservation

Although large islands generally support a richer insect fauna than small islands, many large islands, which are more often inhabited, have lost numerous species because of human activities and introduced organisms. To clarify the consequences of endemic insect conservation on small islands near inhabited islands, we compared the species richness, abundance, and composition of two beetle groups (Coleoptera: Cerambycidae and Mordellidae) captured using Malaise traps among three islands (Chichijima, 24.0 km²; Anijima, 7.85 km²; Nishijima, 0.49 km²) in the oceanic Ogasawara (Bonin) Island group in the northwestern Pacific during June–July 2006 and 2007. Chichijima, the largest island, is inhabited, while Anijima and Nishijima are not. The numbers of cerambycid and mordellid species previously recorded were positively correlated with island area. However, the total numbers of cerambycid and mordellid species we captured in Malaise traps were not correlated with island area because we were unable to collect many species previously documented on Chichijima. The numbers of cerambycid and mordellid species per trap did not differ significantly among islands and years, although the deviance was well explained by the island variable. We captured greater numbers of cerambycid and mordellid individuals on Chichijima than on Anijima and Nishijima, and the numbers of cerambycid and mordellid individuals per trap significantly differed among islands and between years. Redundancy analysis (RDA) showed that the species composition of cerambycids and mordellids differed among the three islands. Whereas endangered species were rarely captured on Chichijima, alien or non-endemic species were frequently collected. Cerambycid and mordellid beetles on Chichijima may have been deleteriously affected by recent forest disturbance and introduced organisms. Therefore, conserving insect fauna on uninhabited island “refugia” is important for preserving the insect diversity of the Ogasawara Islands.     

The influence of tidal marshes on upland groundwater discharge to estuaries

We investigated subsurface hydrology in two fringing tidal marshes and in underlying aquifers in the coastal plain of Virginia. Vertical distributions of hydraulic conductivity, hydraulic head and salinity were measured in each marsh and a nearby subtidal sediment. Discharge of hillslope groundwater into the base of the marshes and subtidal sediment was calculated using Darcy's law. In the marshes, fluxes of pore water across the sediment surface were measured or estimated by water balance methods. The vertical distribution of salt in shoreline sediments was modeled to assess transport and mixing conditions at depth. Hydraulic gradients were upward beneath shoreline sediments; indicating that groundwater was passing through marsh and subtidal deposits before reaching the estuary. Calculated discharge (6 to 10 liters per meter of shoreline per day) was small relative to fluxes of pore water across the marsh surface at those sites; even where discharge was maximal (at the upland border) it was 10 to 50 times less than infiltration into marsh soils. Pore water turnover in our marshes was therefore dominated by exchange with estuarine surface water. In contrast, new interstitial water entering subtidal sediments appeared to be primarily groundwater, discharged from below. The presence of fringing tidal marshes delayed transport and increased mixing of groundwater and solute as it traveled towards the estuaries. Soil-contact times of discharged groundwater were up to 100% longer in marshes than in subtidal shoreline sediments. Measured and modeled salinity profiles indicated that, prior to export to estuaries, the solutes of groundwater, marsh pore water and estuarine surface water were more thoroughly mixed in marsh soils compared to subtidal shoreline sediments. These findings suggest that transport of reactive solutes in groundwater may be strongly influenced by shoreline type. Longer soil-contact times in marshes provide greater opportunity for immobilization of excess nutrients by plants, microbes and by adsorption on sediment. Also, the greater dispersive mixing of groundwater and pore water in marshes should lead to increased availability of labile, dissolved organic carbon at depth which could in turn enhance microbial activity and increase the rate of denitrification in situations where groundwater nitrate is high.     

Genetic Conservation of Insular Populations of Monterey Pine (Pinus Radiata D. Don)

Monterey pine (Pinus radiata D. Don) has only five extant native populations: three disjunct populations along the coast of California, USA and two on Mexican islands. All populations have been influenced by human activity, but the island populations in particular have been affected by introduced biota. On Guadalupe Island, the pine population has suffered drastically from overgrazing by introduced goats. We visited both island populations and described their status, took measurements, and made seed collections. We counted approximately 200 mature pine trees and virtually no seedlings on Guadalupe Island: a reduction of approximately half the population in the last 50 years. The trees are all large (mean diameter of 144 cm) –considerably larger than trees from the other four populations – and arguably near the end of their natural lifespan. The population on Cedros Island is much more robust, with thousands of trees. None sampled were as large as those on Guadalupe Island (mean diameter of 20 cm) and many groves were young and even-aged – presumably the consequence of natural regeneration after a recent fire. Tissue samples from trees on both islands did not show evidence of infection from the pitch canker pathogen, Fusarium circinatum, that has caused significant mortality in the three mainland populations. Caution is recommended in any restoration activity for the Guadalupe Island pines. Inbreeding levels could indicate the need for some planting or seeding intervention but there are also risks associated with this. Natural regeneration – after goat removal – is preferred.     

Water source partitioning among trees growing on shallow karst soils in a seasonally dry tropical climate

The sources of water used by woody vegetation growing on karst soils in seasonally dry tropical regions are little known. In northern Yucatan (Mexico), trees withstand 4–6 months of annual drought in spite of the small water storage capacity of the shallow karst soil. We hypothesized that adult evergreen trees in Yucatan tap the aquifer for a reliable supply of water during the prolonged dry season. The naturally occurring concentration gradients in oxygen and hydrogen stable isotopes in soil, bedrock, groundwater and plant stem water were used to determine the sources of water used by native evergreen and drought-deciduous tree species. While the trees studied grew over a permanent water table (9–20 m depth), pit excavation showed that roots were largely restricted to the upper 2 m of the soil/bedrock profile. At the peak of the dry season, the δ¹⁸O signatures of potential water sources for the vegetation ranged from 4.1 ± 1.1‰ in topsoil to −4.3 ± 0.1‰ in groundwater. The δ¹⁸O values of tree stem water ranged from −2.8 ± 0.3‰ in Talisia olivaeformis to 0.8 ± 1‰ in Ficus cotinifolia, demonstrating vertical partitioning of soil/bedrock water among tree species. Stem water δ¹⁸O values were significantly different from that of groundwater for all the tree species investigated. Stem water samples plotted to the right of the meteoric water line, indicating utilization of water sources subject to evaporative isotopic enrichment. Foliar δ¹³C in adult trees varied widely among species, ranging from −25.3 ± 0.3‰ in Enterolobium cyclocarpum to −28.7 ± 0.4‰ in T. olivaeformis. Contrary to initial expectations, data indicate that native trees growing on shallow karst soils in northern Yucatan use little or no groundwater and depend mostly on water stored within the upper 2–3 m of the soil/bedrock profile. Water storage in subsurface soil-filled cavities and in the porous limestone bedrock is apparently sufficient to sustain adult evergreen trees throughout the pronounced dry season.     

Soil water depletion by Eucalyptus spp. integrated into dryland agricultural systems

Spatial patterns of soil water depletion by Eucalyptus spp. were surveyed to assess the potential of tree belts and short rotation phase farming with trees for groundwater recharge reduction and salinity control. Soils were sampled to depths of up to 10 m in transects perpendicular to 4- to 7-year-old mallee eucalypt belts (Eucalyptus horistes, E. kochii ssp. plenissima, E. loxophleba ssp. lissophloia, E. polybractea) and in a 4 year-old block of E. astringens. Results indicate that the eucalypt species can exploit soil water to depths of at least 8–10 m within 7 years of planting. The root systems of these eucalypts were able to penetrate clayey subsoils with bulk densities of up to 2.0 g cm⁻³. Leaf area indexes of tree belts were 2–10 times greater than those predicted for natural vegetation, probably as a result of exploiting a greater amount of soil water stored under the agricultural system. The lateral influence of mallee belts, as indicated by soil water contents that were depleted to wilting point, ranged from 15–42 m. The resulting dry soil zone provided an effective barrier to groundwater recharge by incident rainfall thereby lessening the risk of salinisation in the agricultural landscape. The width of this barrier to recharge was predicted to range from 7 m to 54 m based on leaf area.     

The influence of tidal marshes on upland groundwater discharge to estuaries

We investigated subsurface hydrology in two fringing tidal marshes and in underlying aquifers in the coastal plain of Virginia. Vertical distributions of hydraulic conductivity, hydraulic head and salinity were measured in each marsh and a nearby subtidal sediment. Discharge of hillslope groundwater into the base of the marshes and subtidal sediment was calculated using Darcy's law. In the marshes, fluxes of pore water across the sediment surface were measured or estimated by water balance methods. The vertical distribution of salt in shoreline sediments was modeled to assess transport and mixing conditions at depth. Hydraulic gradients were upward beneath shoreline sediments; indicating that groundwater was passing through marsh and subtidal deposits before reaching the estuary. Calculated discharge (6 to 10 liters per meter of shoreline per day) was small relative to fluxes of pore water across the marsh surface at those sites; even where discharge was maximal (at the upland border) it was 10 to 50 times less than infiltration into marsh soils. Pore water turnover in our marshes was therefore dominated by exchange with estuarine surface water. In contrast, new interstitial water entering subtidal sediments appeared to be primarily groundwater, discharged from below. The presence of fringing tidal marshes delayed transport and increased mixing of groundwater and solute as it traveled towards the estuaries. Soil-contact times of discharged groundwater were up to 100% longer in marshes than in subtidal shoreline sediments. Measured and modeled salinity profiles indicated that, prior to export to estuaries, the solutes of groundwater, marsh pore water and estuarine surface water were more thoroughly mixed in marsh soils compared to subtidal shoreline sediments. These findings suggest that transport of reactive solutes in groundwater may be strongly influenced by shoreline type. Longer soil-contact times in marshes provide greater opportunity for immobilization of excess nutrients by plants, microbes and by adsorption on sediment. Also, the greater dispersive mixing of groundwater and pore water in marshes should lead to increased availability of labile, dissolved organic carbon at depth which could in turn enhance microbial activity and increase the rate of denitrification in situations where groundwater nitrate is high.     

Major ion chemistry of the lower Boro River, Okavango Delta, Botswana

SUMMARY. 1. The composition of surface waters of the Boro River, Okavango Delta, Botswana, was determined on five occasions during 1989–1990. The waters could be characterized as caicium—sodium—bicarbonate, with moderate alkalinity, and moderate to high amounts of silica. Mean values of the major ions, in mg1-⁻¹, were: Ca²⁺ 4.8, Mg²⁺ 1.3, Na⁺ 3.9, K⁺ 2.7, HCO₃⁻ 27, Cl⁻ 1.0 and SiO₂ 38; pH c. 7.
2. The relative ionic composition of the waters changed gradually from the inlet in Seronga to the outlet at Boro/Thamalakane junction. There was a gradient of increasing concentrations of solutes resulting from the evaporative concentration of the waters in this semi-arid region with an almost permanent water deficit, and the total dissolved solutes increased from c. 30 to 95 mg 1-⁻¹
3. There were seasonal differences in solute concentrations, connected with the rainfall pattern and climatic cycle and an ill-defined relation with discharge. The chemical spectrum of the Boro, and the freshness of its waters, can be attributed to the major influence of precipitation within the basin and selective removal of solutes.
4. Chemical equilibrium models indicate that amorphous silica and sepiolite are likely to precipitate from the Boro waters. Carbonate and silicate systems are responsible for the selective removal of solutes in the swamps, islands, or flood plains, as proposed by others in recent studies, but not in the Boro River waters.     

Macaca nigra on Bacan Island,Indonesia: Its morphology,distribution, and present habitat

We studied the monkeys inhabiting Bacan Island on the Maluku Archipelago, Indonesia. We compared them morphologically with Sulawesi black macaques (Macaca nigra).We also assessed their distribution on the island and on neighboring islands and the influence of human activity on them. We found that the Bacan black macaques are essentially similar to Sulawesi macaques; the variations in each locality are comparable. The monkeys probably inhabit the whole island or, at least, the lower forest thereon. Human activities — cultivation, logging, catching and killing monkeys — have not much affected their population. Thus they constitute a good stock of Macaca nigra,though the main population in Sulawesi is endangered.