Fine root respiration in the mangrove Rhizophora mangle over variation in forest stature and nutrient availability

Root respiration uses a significant proportion of photosynthetically fixed carbon (C) and is a globally important source of C liberated from soils. Mangroves, which are an important and productive forest resource in many tropical and subtropical countries, sustain a high ratio of root to shoot biomass which may indicate that root respiration is a particularly important component in mangrove forest carbon budgets. Mangroves are often exposed to nutrient pollution from coastal waters. Here we assessed the magnitude of fine root respiration in mangrove forests in Belize and investigated how root respiration is influenced by nutrient additions. Respiration rates of excised fine roots of the mangrove, Rhizophora mangle L., were low (4.01 +/- 0.16 nmol CO2 g(-1) s(-1)) compared to those measured in temperate tree species at similar temperatures. In an experiment where trees where fertilized with nitrogen (N) or phosphorus (P) in low productivity dwarf forests (1-2 m height) and more productive, taller (47 m height) seaward fringing forests, respiration of fine roots did not vary consistently with fertilization treatments or with forest stature. Fine roots of taller fringe trees had higher concentrations of both N and P compared to dwarf trees. Fertilization with P enhanced fine root P concentrations in both dwarf and fringe trees, but reduced root N concentrations compared to controls. Fertilization with N had no effect on root N or P concentrations. Unlike photosynthetic C gain and growth, which is strongly limited by P availability in dwarf forests at this site, fine root respiration (expressed on a mass basis) was variable, but showed no significant enhancements with nutrient additions. Variation in fine root production and standing biomass are, therefore, likely to be more important factors determining C efflux from mangrove sediments than variations in fine root respiration per unit mass.

Water and nutrient availabilities do not affect lignotuber growth and sprouting ability of three eucalypt species of south-eastern Queensland

Shoot biomass and lignotuber size of seedlings of three eucalypt species, Eucalyptus acmenoides Schauer, E. siderophloia Benth. and Corymbia variegata [syn. E. maculata (F. Muell.)K. D. Hill and L. A. S. Johnson], were measured for glasshouse-grown seedlings established under two water and nutrient regimes. Seedlings were subjected to shoot removal (clipping) at ages from 9 to 19 weeks, and transferred to the high water treatment for a further 8 weeks to assess shoot emergence from lignotubers. Seedling shoot biomass was greater in both the high than the low nutrient and water treatments, but lignotuber diameter was not affected significantly. C. variegata seedlings had the largest lignotuber diameters, followed by E. siderophloia and E. acmenoides, respectively. Although growth of shoots was influenced by nutrient availability, results suggest that species differences in the growth of lignotubers was less affected. It is suggested that lignotuber growth was strongly influenced by genotype. More than 70% of C. variegata seedlings clipped at 9 weeks sprouted, compared with only 5 and 10% of seedlings of E. siderophloia and E. acmenoides, respectively. All C. variegata seedlings sprouted after being clipped at 19 weeks, but < 80% of E. siderophloia and < 60% of E. acmenoides sprouted when clipped at the same age. It was concluded that seedlings forming part of the regeneration stratum in dry sclerophyll forests need to be protected from damage for at least 4 months (for C. variegata) or at least 6 months (for E. siderophloia and E. acmenoides) if they are to survive by sprouting from lignotubers.

Variation in hydraulic conductivity of mangroves: influence of species, salinity, and nitrogen and phosphorus availability

We investigated how species identity and variation in salinity and nutrient availability influence the hydraulic conductivity of mangroves. Using a fertilization study of two species in Florida, we found that stem hydraulic conductivity expressed on a leaf area basis (K-leaf) was significantly different among species of differing salinity tolerance, but was not significantly altered by enrichment with limiting nutrients. Reviewing data from two additional sites (Panama and Belize), we found an overall pattern of declining leaf-specific hydraulic conductivity (K-leaf) with increasing salinity. Over three sites, a general pattern emerges, indicating that native stem hydraulic conductivity (K-h) and K-leaf are less sensitive to nitrogen (N) fertilization when N limits growth, but more sensitive to phosphorus (P) fertilization when P limits growth. Processes leading to growth enhancement with N fertilization are probably associated with changes in allocation to leaf area and photosynthetic processes, whereas water uptake and transport processes could be more limiting when P limits growth. These findings suggest that whereas salinity and species identity place broad bounds on hydraulic conductivity, the effects of nutrient availability modulate hydraulic conductivity and growth in complex ways.

Benthic microalgae in coral reef sediments of the southern Great Barrier Reef, Australia

The abundance and productivity of benthic microalgae in coral reef sediments are poorly known compared with other, more conspicuous (e.g. coral zooxanthellae, macroalgae) primary producers of coral reef habitats. A survey of the distribution, biomass, and productivity of benthic microalgae on a platform reef flat and in a cross-shelf transect in the southern Great Barrier Reef indicated that benthic microalgae are ubiquitous, abundant (up to 995.0 mg chlorophyll (chl) a m(-2)), and productive (up to 110 mg O-2 m(-2) h(-1)) components of the reef ecosystem. Concentrations of benthic microalgae, expressed as chlorophyll a per surface area, were approximately 100-fold greater than the integrated water column concentrations of microalgae throughout the region. Benthic microalgal biomass was greater on the shallow water platform reef than in the deeper waters of the cross-shelf transect. In both areas the benthic microalgal communities had a similar composition, dominated by pennate diatoms, dinoflagellates, and cyanobacteria. Benthic microalgal populations were potentially nutrient-limited, based on responses to nitrogen and phosphorus enrichments in short-term (7-day) microcosm experiments. Benthic microalgal productivity, measured by O-2 evolution, indicated productive communities responsive to light and nutrient availability. The benthic microalgal concentrations observed (92-995 mg chl a m(-2)) were high relative to other reports, particularly compared with temperate regions. This abundance of productive plants in both reef and shelf sediments in the southern Great Barrier Reef suggests that benthic microalgae are key components of coral reef ecosystems.

Nitrogen, phosphorus, silica, and carbon in Moreton Bay, Queensland, Australia: Differential limitation of phytoplankton biomass and production

Subtropical estuaries have received comparatively little attention in the study of nutrient loading and subsequent nutrient processing relative to temperate estuaries. Australian estuaries are particularly susceptible to increased nutrient loading and eutrophication, as 75% of the population resides within 200 km of the coastline. We assessed the factors potentially limiting both biomass and production in one Australian estuary, Moreton Bay, through stoichiometric comparisons of nitrogen (N), phosphorus (P), silicon (Si), and carbon (C) concentrations, particulate compositions, and rates of uptake. Samples were collected over 3 seasons in 1997-1998 at stations located throughout the bay system, including one riverine endmember site. Concentrations of all dissolved nutrients, as well as particulate nutrients and chlorophyll, declined 10-fold to 100-fold from the impacted western embayments to the eastern, more oceanic-influenced regions of the bay during all seasons. For all seasons and all regions, both the dissolved nutrients and particulate biomass yielded N : P ratios < 6 and N : Si ratios < 1. Both relationships suggest strong limitation of biomass by N throughout the bay. Limitation of rates of nutrient uptake and productivity were more complex. Low C : N and C : P uptake ratios at the riverine site suggested light limitation at all seasons, low N : P ratios suggested some degree of N limitation and high N : Si uptake ratios in austral winter suggested Si limitation of uptake during that season only. No evidence of P limitation of biomass or productivity was evident.

Limitations on bioassays in macronutrient deficiency determination

Short-term nutrient bioassays can be used to assess labile nutrient availability in soils. These bioassays rely on a high number of plants and small soil volumes to exploit labile soil resources rapidly and assess potential nutrient deficiency. A comparison of the Neubauer bioassay with conventional pot trial assessment of P and S availability in a Yellow Kurosol was undertaken. Changes in labile soil nutrients and enzyme activity after bioassay assessment were also measured. The Neubauer bioassay was able to detect increased labile P availability following P fertiliser application to the soil. This corresponded with response to added P in a longer-term pot trial using maize. As expected, phosphatase activity increased following the bioassay and labile P was depleted by the plants. However, although a longer-term pot trial demonstrated the Yellow Kurosol was responsive to S fertilisation, labile S pools were sufficiently large that the short-term Neubauer bioassay detected no difference in S availability to plants. Both soil sulphatase activity and labile soil S were elevated following the bioassay. The short period of contact between the roots of the bioassay and the soil may have limited S uptake and therefore the ability of the bioassay to identify a S responsive soil. When using bioassay techniques to assess labile nutrient availability, it is critical that the size of the labile nutrient pool present be considered for each element, and that the period of contact between the bioassay and soil being tested is long enough for plant uptake to lower the nutrient supply to a level that limits further uptake.

Food choice in Siganus fuscescens: influence of macrophyte nutrient content and availability

Using multiple-choice feeding experiments, the selection of six species of macrophytes by the herbivorous rabbitfish Siganus fuscescens was examined. The rabbitfish showed distinct food choice in the laboratory; however, selection of macrophytes by S. fuscescens was not related to their absolute nutrient content (nitrogen, carbon, energy and ash free dry mass). Nutrient assimilation estimates showed that the macrophytes which were most preferred were those that S. fuscescens assimilated best. In S. fuscescens, the macrophytes that were preferred passed through the gut significantly faster than the less preferred species. Gut transit time had a significant effect on the absolute value of a food item in terms of net nutrient gain per unit time. This study showed that food value could be inferred from the absolute nutrient content of the macrophytes. Thus both the ability to assimilate nutrients as well as the absolute nutrient content of macrophytes must be quantified when assessing food value. (C) 2004 The Fisheries society of the British Isles.

Food and nutrient consumption patterns in third trimester Thai-Muslim pregnant women in rural Southern Thailand

The aims of this study were to determine food and nutrient intakes and the socio-economic factors influencing food and nutrient intakes of rural Thai-Muslim women in the third trimester of pregnancy. The study was conducted in Pattani province, Thailand, where 166 women were interviewed between 32 and 40 weeks gestation. A questionnaire. including a Food Frequency Questionnaire was used. Data on food items were compiled into the five basic Thai food groups, and food intakes were computed into macro and micro-nutrients. Mean weight intake of each of the five groups was below the recommended level for pregnant Thai women. Mean intake of niacin, vitamin A (RE) and vitamin C were above the recommended Thai level. Thiamin, calcium. phosphorus and iron intakes were lower than 50% of recommended levels. Intakes of the five food groups were not associated with socio-economic status, although total non-haem iron intake was associated with level of education. Under-consumption of food and nutrients among pregnant women in the study area was due to poor education. poverty and food availability. Integrated strategies should be considered to promote increased intakes to meet nutrient recommendations.