Divide or broadcast: Interrelation of asexual and sexual reproduction in a population of the fissiparous hermaphroditic seastar Nepanthia belcheri (Asteroidea: Asterinidae)

Asexual and sexual reproduction were studied in an intertidal population of Nepanthia belcheri (Perrier) at Townsville, Queensland, Australia, by regular sampling over a year (March 1976-March 1977) and by histological analysis of gonads. Fission reached a peak in early winter (April–June), when about 45% of the population showed evidence of recent fission. Propensity for fission was unrelated to longest arm length. Seven-armed seastars predominated in the population and these underwent fission in two stages to produce one 3-armed fragment and two 2-armed fragments. Fission planes were not related to numbers or positions of madreporites. Hermaphroditism was a normal sexual condition in the population. Almost all gonads contained oocytes, but some gonads functioned as ovaries (without spermatogenic tissue) while others functioned as testes. Seastars with mature ovaries were significantly larger than those with mature testes, indicating protandry, as in other hermaphroditic asteroids. However, fission complicates the pattern of gonad development by causing regression or retardation of gonads and by apparently having a masculinizing effect, so that ovaries may change to testes in fission products. There was a period of sexual reproduction in early summer (October–November). This followed the period of intense fission and regeneration, and a population change from predominately functional females to males. Consequently there was an extreme imbalance against mature females at sexual reproduction, further reducing potential fecundity. Thus, sexual reproduction was very subordinate to fission as the means of recruitment. The 450 m diam eggs probably give rise to pelagic lecithotrophic development and, if this is the case, N. belcheri retains the advantage of complementing reliable recruitment from fission with a dispersive phase. The combination of fission and hermaphroditism is particularly advantageous for a very sparse dispersal of larvae, as a functionally dioecious population may develop from one larva settling in a new locality.     

Water Quality Functions of Riparian Forest Buffers in Chesapeake Bay Watersheds

/ Maryland, Virginia, and Pennsylvania, USA, have agreed to reduce nutrient loadings to Chesapeake Bay by 40% by the year 2000. This requires control of nonpoint sources of nutrients, much of which comes from agriculture. Riparian forest buffer systems (RFBS) provide effective control of nonpoint source (NPS) pollution in some types of agricultural watersheds. Control of NPS pollution is dependent on the type of pollutant and the hydrologic connection between pollution sources, the RFBS, and the stream. Water quality improvements are most likely in areas of where most of the excess precipitation moves across, in, or near the root zone of the RFBS. In areas such as the Inner Coastal Plain and Piedmont watersheds with thin soils, RFBS should retain 50%-90% of the total loading of nitrate in shallow groundwater, sediment in surface runoff, and total N in both surface runoff and groundwater. Retention of phosphorus is generally much less. In regions with deeper soils and/or greater regional groundwater recharge (such as parts of the Piedmont and the Valley and Ridge), RFBS water quality improvements are probably much less. The expected levels of pollutant control by RFBS are identified for each of nine physiographic provinces of the Chesapeake Bay Watershed. Issues related to of establishment, sustainability, and management are also discussed.KEY WORDS: Riparian forest buffers; Chesapeake Bay; Nonpoint source pollution; Nitrogen; Phosphorus; Sediment     

Nutrient limitation in the giant clam-zooxanthellae symbiosis: effects of nutrient supplements on growth of the symbiotic partners

The effect of ammonium (5, 10 M N) and phosphate (2, 5, 10 M P) on the growth of the giant clam Tridacna gigas and its symbiotic dinoflagellate Symbiodinium sp. was examined. A 3 mo exposure to these nutritients significantly increased the N or P composition of the soft tissues, as reflected in a corresponding change in C:N:P ratio. Furthermore, exposure to N or N+P markedly increased the amount of soft tissue, but P alone did not, demonstrating that increased availability of inorganic nitrogen enhances tissue growth of the clam host. With addition of N, or N+P, there was a significant increase in the total number of zooxanthellae per clam, with a corresponding decrease in chlorophyll a (chl a) content per zooxanthella. However, only with N+P was there an increase in the zooxanthellae mitotic index. The inverse relationship between zooxanthellae number and chl a per zooxanthella is consistent with phytoplankton studies indicating conditions of nutrient-limitation. Furthermore, the unaffected C:N:P composition of the zooxanthellae and their relatively low specific-growth rates (4 to 10%) also suggest that they are nutrient-limited in vivo. In particular, their high mean C:N:P ratio of 303:52:1 indicates that, relative to C, they are much more depleted in P and less in N than are free-living phytoplankton. Furthermore, polyphosphates (phosphate reserves) were undetectable, and the activity levels of acid phosphatase in the zooxanthellae were relatively high and not influenced by the host's exposure to increased P concentrations in the sea water, implicating the clam host in active regulation of P availability to its symbiotic algae. This is strong evidence that N-limitation of clam zooxanthellae is a function of the availability of ammonium to the symbiosis while, irrespective of nutrient levels in sea water, clam zooxanthellae still show characteristics of P-limitation.     

The FAIR model: A tool to analyse environmental and costs implications of regimes of future commitments

This article describes the policy decision-support tool, FAIR, to assess the environmental and abatement costs implications of international regimes for differentiation of future commitments. The model links long-term climate targets and global reduction objectives with regional emission allowances and abatement costs, accounting for the Kyoto Mechanisms used. FAIR consists of three sub-models: a simple climate model, an emission-allocation model and a cost model. The article also analyses ten different rule-based emission allocation schemes for two long-term concentration stabilisation targets for greenhouse gases. This analysis shows that evaluating regimes requires not only an assessment of the initial allocation, but also of the distribution of abatement costs and the impacts from emissions trading. The Multi-Stage approach (with a gradual increase of Parties adopting emission intensity or reductions targets) and the Triptych approach (with sectoral targets for all Parties) seem to provide the best prospects for most of the Parties when compared to the other allocation schemes analysed.     

Soil N balance as affected by soybean maturity class in the Guinea savanna of Nigeria

Legume–cereal rotation may reduce the fertilizer requirement of the cereal crop and we hypothesize that the benefit depends on the maturity class of the soybean. Field trials were therefore conducted in 1995 in four Guinea savanna sites to monitor the effect of soybean (Glycine max (L.) Merrill) cultivation on the N balance of the soil. In trial 1, an early (TGx1485-1D) and a late (TGx1670-1F) soybean were grown to maturity along with a maize (Zea mays L.) reference plot. In trial 2, six varieties of soybean (early: TGx1485-1D, TGx1805-2E and TGx1681-3F; medium: TGx1809-12E and TGx923-2E; late: TGx1670-1F) were grown to maturity along with a reference maize plot. The total nitrogen (N) content, aboveground N₂ fixed, and N remaining in the stover were higher in the medium and the late varieties than in early varieties. Also, the early varieties had higher nitrogen harvest indices (81–84%) than medium and late varieties (74–79%). From the N balance calculation, it was found that medium and late maturing soybean resulted in an addition of 4.2 kg N ha⁻¹ to the soil, whereas the early maturing varieties resulted in depletion of the soil N reserve by 5.6 kg N ha⁻¹ (P<0.05). On average, among the medium and late varieties, late maturing TGx923-2E resulted in an addition of 9.5 kg N ha⁻¹ to the soil. When the stover was not returned to the field, early soybean resulted in more negative N balance than the medium and late soybean (P<0.05). Therefore, planting an early variety of soybean for one season resulted in net depletion of soil N, even when the soybean residues were returned to the soil and N₂ fixed in the roots and N in the fallen leaf litter were included in the N balance calculations. Contrary to this, planting medium and late soybean for one season resulted in an addition of N to the soil. Therefore, medium and late soybean should be used as a preceding crop in legume–cereal rotation, if possible, to minimize or avoid depletion of soil N by early varieties of soybean.     

Carbon budgets in temperate anthozoan-dinoflagellate symbioses

Carbon budgets were modelled for temperate anthozoan-dinoflagellate symbioses involving the sea anemones Cereus pedunculatus (Pennant), Anthopleura ballii (Cocks) and Anemonia viridis (Forskäl), and the zoanthid Isozoanthus sulcatus (Gosse). Irradiance regimes experienced at 1.5 and 9 m on sunny and cloudy days in summer were assumed. Photosynthetic capacity (P _max gross) and efficiency () were considerably higher in I. sulcatus than in the other Anthozoa. P _max gross and also differed in A. viridis from different localities. At 1.5 m on sunny days, zooxanthellae would require 1.80 to 5.89% of the carbon fixed in photosyn-thesis for respiration and growth, and translocate the remainder (94.11 to 98.20%) to the host. Productivity would decrease with increasing depth and cloud cover, resulting in a decrease in the potential availability of carbon for translocation. At 9 m on cloudy days, 37.82 to 87.84% of the carbon fixed in photosynthesis would be required for zooxanthella respiration and growth in C. pedunculatus, Anthopleura ballii and Anemonia viridis, leaving just 12.16 to 62.18% for translocation; the translocation rate would still exceed 95% in I. sulcatus. The potential contribution of zooxanthellae to the host's daily respiratory carbon requirements (CZAR) would be 72.6 and 72.1% in Anthopleura ballii and C. pedunculatus, respectively, at 1.5 m on sunny days, and would decrease to just 2.1 and 0.7%, respectively, at 9 m on cloudy days. These Anthozoa therefore require a heterotrophic source of carbon to survive. The CZAR in Anemonia viridis from different locations would be 140.6 to 142.9% at 1.5 m on sunny days, but would be <100% under the other assumed irradiance regimes. The CZAR in I. sulcatus would be 181.5% at 1.5 m on sunny days, and would only be <100% when at 9 m on cloudy days. Under favourable conditions, A. viridis and I. sulcatus are potentially autotrophic and may have surplus carbon available (15.69 to 43.89% of the gross photosynthetic production) for tissue biosynthesis, reproduction and storage. However, when field conditions are considered on an annual basis, the general need for heterotrophically-derived carbon in temperate Anthozoa is suggested.     

Clonal extent,apical dominance and networking features in the phalanx angiosperm <Emphasis Type="Italic">Zostera noltii</Emphasis> Hornem.

Disaggregating seagrass meadows and studying its components separately (clones, ramets, shoots) can provide us insights on meadow dynamics and growth patterns. The clonal growth, dependent upon clonal rules may regulate and impose constraints to plant architecture and, therefore, determine how individual clones evolve into the environment. In order to investigate the relationship between clonal growth rules and clone architecture, the belowground network architecture of single-clones of the seagrass Zostera noltii was studied. Networks were traced in situ after washing out the overlying sediment, and network characteristics were measured using digital analysis: area covered by clone, total rhizome length, type of rhizomatic axes (main, secondary, tertiary, quaternary), number and length of the internodes, branching angles and branching frequencies. This approach revealed that Z. noltii is able to develop into large clones integrating up to 300 internodes, 676 cm of rhizome, 208 shoots and 4,300 cm² of plant area. Internodal length depended on both, the distance to the apical shoot (time effect) and the axes type (apical dominance effect). However, average branching angle was independent of axis type (average 58.3 ± 0.75), but varied significantly depending on the distance from the apical shoot. This average branching angle allows Z. noltii maximize the rate of centrifugal expansion, maintaining a high density in colonized areas to produce close stands but also minimizing the investment in belowground biomass and ramets overlapping. The clonal architecture of Z. noltii seems to be regulated by the interaction of both, apical dominance strength and clonal integration distance. Moreover, clonal growth rules and growth pattern seem to constrain clonality through (clonal) plant architecture regulations (i.e. branching is restricted in secondary axes, similar average branching angles regardless the axes, the higher the distance to the apex the higher the number of internodes in secondary axes, shorter internodes in secondary and tertiary axes). Future research efforts should focus on how these complex relationships between apical dominance and clonal integration interact to elucidate the temporal (seasonal) and spatial scales of both processes and the outcome at the plant architectural level.