KINETICS OF NITROGEN-15 LABELLED AMMONIUM UPTAKE BY CAULERPA CUPRESSOIDES (CHLOROPHYTA)1,2,3

The incorporation of ¹⁵NH₄⁺ as a function of time and concentration was used to estimate ammonium uptake by Caulerpa cupressoides (West) C. Agardh, taken from its habitat on the sediments of Tague Bay lagoon, St. Croix, U.S. Virgin Islands. ¹⁵N-based uptake rates followed Michaelis-Menten type saturation kinetics; the maximum uptake rate was 8.7 ± 3.0 μmol N/g dry wt·h at 26° C and the half-saturation constant was 48 ± 10 μM (X?± SE). The high half-saturatiaon constant reflects the dependence of Caulerpa on sediment pore waters as a nitrogen source. Calculations of uptake from isotope time course data were compared to estimates made from ammonium depletion. More ammonium disappeared than could be accounted for by the incorporation of ¹⁵N in Caulerpa, and isotope dilution of the ammonium pool is shown not to be responsible for underestimates of uptake, primarily because large ¹⁵N additions (40–300 μM) were used. It is suggested that either (1) a secondary ammonium sink such as wall sorption or bacterial uptake significantly influenced ammonium concentrations, or (2) ¹⁵N was lost as labelled dissolved organic nitrogen or volatilized during ¹⁵N sample preparation.     

Decomposition and nitrogen dynamics of turtle grass (<Emphasis Type="Italic">Thalassia testudinum</Emphasis>) in a subtropical estuarine system

Seagrass beds are pivotal in the functioning of coastal ecosystems in terms of productivity, organic matter turnover and nutrient cycling. Aiming to document decay and nitrogen (N) dynamics of turtle grass (Thalassia testudinum) in a subtropical estuarine system, decomposition patterns of leaves and rhizomes were characterized and compared. Nitrogen usage during decomposition of tissues, and of live tissues and epiphytes growing on live leaves, was also quantified and compared. Stable isotope ratios allowed tracing N within the seagrass bed, following N incorporation into seagrass tissues from the surrounding media (water, sediment). Leaves had a higher N content and decomposed at a faster rate (~6.4 times) compared to rhizomes. Leaching of soluble materials explain the rapid (0–3 days) initial mass loss of leaves (20%) and rhizomes (18%); with a loss of 85 and 56%, respectively, by the end of the study (77 days). Overall, leaves released N while rhizomes immobilized it. Nitrogen concentration was significantly different among live tissues. The main source of N for both seagrass tissues was the sediment, and water column for epiphytes. Differences in decomposition rates among seagrass tissues can be explained by the quality of the substrate and its susceptibility to microbial use. Seagrass leaves and rhizomes are equally important in taking up nutrients from either the water column or the sediments. This study provides a platform to study energy and matter transfers through detrital foodwebs linked to seagrass meadows.     

Material exchange and food web of seagrass beds in the Sylt-Rømø Bight: how significant are community changes at the ecosystem level?

Material exchange, biodiversity and trophic transfer within the food web were investigated in two different types of intertidal seagrass beds: a sheltered, dense Zostera marina bed and a more exposed, sparse Z. noltii bed, in the Northern Wadden Sea. Both types of Zostera beds show a seasonal development of above-ground biomass, and therefore measurements were carried out during the vegetation period in summer. The exchange of particles and nutrients between seagrass beds and the overlying water was measured directly using an in situ flume. Particle sedimentation [carbon (C), nitrogen (N) and phosphorus (P) constituents] from the water column prevailed in dense seagrass beds. In the sheltered, dense seagrass bed, a net particle uptake was found even on windy days (7–8 Beaufort). Dissolved inorganic N and orthophosphate were mainly taken up by the dense seagrass bed. At times of strong winds, nutrients were released from the benthic community to tidal waters. In a budget calculation of total N and total P, the dense seagrass beds were characterised as a material sink. The seagrass beds with sparse Z. noltii were a source of particles even during calm weather. The uptake of dissolved inorganic N in the sparse seagrass bed was low but significant, while the uptake of inorganic phosphate and silicate by seagrasses and their epiphytes was exceeded by release processes from the sediment into the overlying water. Estimates at the ecosystem level showed that material fluxes of seagrass beds in the Sylt-Rømø Bight are dominated by the dense type of Zostera beds. Therefore, seagrass beds act as a sink for particles and for dissolved inorganic nutrients. During storms, seagrass beds are distinct sources for inorganic nutrients. The total intertidal area of the Sylt-Rømø Bight could be described as a sink for particles and a source for dissolved nutrients. This balance of the material budget was estimated by either including or excluding seagrass beds. Including the subtidal part, the function of the ecosystem as a source for particles increased, supposing that all seagrass beds were lost from the area. During the vegetation period, seagrass beds act as a storage compartment for material accumulated in the living biomass of the community. There was great biodiversity among the plant and animal groups found in intertidal seagrass beds of the Sylt-Rømø Bay, representing 50–86% of the total number of species investigated, depending on the particular group. Since most species are not exclusively seagrass residents, the loss of intertidal seagrass beds would be of minor importance for biodiversity at the ecosystem level. Food web structure in seagrass beds is different from other intertidal communities. Primary production and detritus input is high, but secondary production is similar to that of unvegetated areas, although the relative importance of the trophic guilds is different. The loss of seagrass beds leads to profound alterations in the food web of the total ecosystem. Historical as well as recent changes in material fluxes and energy flow due to man-made alterations to the ecosystem are discussed.     

Effects of sediment nutrients on seagrasses: Literature review and mesocosm experiment

A review of the literature shows that seagrass growth, abundance and morphology are strongly linked to available nutrient resources. In north temperate climates and in habitats with terrigenous sediments, nitrogen limitation occurs in both intertidal and subtidal beds. Typically, seagrasses growing in terrigenous sediments have ample phosphorus but lack nitrogen, and the plants' chemical composition is depleted in nitrogen. However, seagrasses occurring in tropical environments and carbonate sediments appear to experience phosphorus limitation due to binding of phosphate in the sediments. Thus, it is the sediment geochemistry in seagrass beds that is important in determining the limiting nutrient to seagrass growth.Examination of the literature indicates that field research on seagrass relationships involves too many interactive factors to be able to say certainly that any one plant characteristics is caused by any one environmental factor. Using mesocosms (partially enclosed outdoor experimental set-ups) one environmental factor can be changed between the treatment tank and the control. Therefore, we can determine experimentally that a plant characteristic is affected by a particular altered environmental factor. Experimental mesocosms used to grow eelgrass, Zostera marina L., in substrata of varied nitrogen composition showed the dramatic effect of insufficient nitrogen on eelgrass growth, abundance and leaf morphology. Additionally, eelgrass leaf tissue from low ammonium sand substratum is significantly depleted in nitrogen, demonstrating the supposition that nitrogen is limiting in terrigenous environments.     

Seasonal variation in rates of heterotrophic nitrogen fixation (acetylene reduction) in Zostera noltii meadows and uncolonised sediments of the Bassin d'Arcachon, south-west France

Nitrogen fixation (acetylene reduction) rates were measured over an annual cycle in meadows of the seagrass Z. noltii and uncolonised sediments of the Bassin d'Arcachon, south-west France, using both slurry and whole core techniques. Measured rates using the slurry technique in Z. noltii colonised sediments were consistently higher than those determined in isolated cores. This was probably due to the release of labile organic carbon sources during preparation of the slurries. Thus, in colonised sediments the whole core technique may provide a more accurate estimate of in situ activity. Acetylene reduction rates measured by the whole core technique in colonised sediments were 1.8 to 4-fold greater, dependent upon the season, in the light compared with those measured in the dark, indicating that organic carbon released by the plant roots during photosynthesis was an important factor regulating nitrogen fixation. In contrast acetylene reduction rates in uncolonised sediments were independent of light.Addition of sodium molybdate, a specific inhibitor of sulphate reduction inhibited acetylene reduction activity in Z. noltii colonised sediments by > 80% as measured by both slurry and whole core techniques irrespective of the light regime, throughout the year inferring that sulphate reducing bacteria (SRB) were the dominant component of the nitrogen fixing microflora. A mutualistic relationship between Z. noltii and nitrogen fixing SRB in the rhizosphere, based on the exchange of organic carbon and fixed nitrogen is proposed. In uncolonised sediments sodium molybdate initially severely inhibited acetylene reduction rates, but the level of this inhibition declined over the course of the year. These data indicate that the nitrogen fixing SRB associated with the Zostera roots and rhizomes were progressively replaced by an aerobic population of nitrogen fixers associated with the decomposition of this recalcitrant high C:N ratio organic matter.Acetylene and sulphate reduction rates in the seagrass beds showed distinct summer maxima which correlated with a reduced availability of NH ₄ ⁺ in the sediment and the growth cycle of Z. noltii in the Bassin. Overall, these data indicate that acetylene reduction (nitrogen fixation) activity in the rhizosphere of Z. noltii was regulated both by release of organic carbon from the plant roots and maintenance of low ammonium concentrations in the root zone due to efficient ammonium assimilation.Nitrogen fixation rates determined from acetylene reduction rates measured by the whole core technique ranged from 0.1 to 7.3 mg N m^–2 d^–1 in the Z. noltii beds and between 0.02 and 3.7 mg N m^–2 d^–1 in uncolonised sediments, dependent upon the season. Nitrogen fixation in the rhizosphere of Z. noltii was calculated to contribute between 0.4 and 1.1 g N m^–2 y^–1 or between 6.3 and 12% of the annual fixed nitrogen requirement of the plants. Heterotrophic nitrogen fixation therefore represents a substantial local input of fixed nitrogen to the sediments of this shallow coastal lagoon and contributes to the overall productivity of Z. noltii in this ecosystem.     

Space competition between seagrass and Caulerpa prolifera (Forsskaal) Lamouroux following simulated disturbances in Lassing Park, FL

An experimental study was conducted in Tampa Bay, FL to examine the response to disturbance of two co-occurring subtidal plants: the alga Caulerpa prolifera and the seagrass Halodule wrightii (Ascherson). Some recent studies have called into question the assumption that fast-growing rhizoidal Caulerpa species have the potential to outcompete and rapidly replace local seagrasses. In the Fall of 2002 an abrupt appearance of Caulerpa prolifera was noted in a shallow embayment in Tampa Bay previously dominated by seagrasses. Natural disturbance events were simulated by excavating 0.5 × 0.5 m plots in an area with monospecific C. prolifera and mixed C. prolifera and H. wrightii. Above and below-ground biomass were removed, and recovery of above-ground cover into the newly created gaps was monitored over 15 months. In addition to measuring the recovery of both species, the spatial pattern of Caulerpa recovery from the simulated disturbances was also analyzed. Simulated gaps were rapidly (5-8 months, depending on sampling resolution) and exclusively reoccupied by C. prolifera, with the recovery occurring predominantly via lateral expansion from gap edges rather than colonization by fragments. Therefore, while rhizoidal algae may or may not be able to supplant existing seagrasses by overgrowth or other forms of direct competition, disturbance events that remove seagrass and create bare areas may allow C. prolifera to replace seagrasses over time via preemption of space should an algal bloom such as this be persistent.     

Predicting risks of invasion of macroalgae in the genus Caulerpa in Florida

There is worldwide concern about the aquarium strain of the green alga Caulerpa taxifolia (Vahl) C. Agardh that was introduced to the Mediterranean Sea in 1984. Since that time, it has flourished and now covers thousands of hectares of near-shore waters. More recently, aquarium strains of C. taxifolia invaded southern California and Australian waters. Our goal was to evaluate potential invasion of C. taxifolia to Florida’s coastal waters. We looked for evidence of C. taxifolia—aquarium strain, as well as the present distribution of all species of Caulerpa, in Florida’s near-shore waters. We surveyed 24 areas in six zones along the Floridian coastline, and evaluated the association of potential indicators for the presence of Caulerpa. Latitude, presence of seagrass beds, human population density, and proximity to marinas were the four variables simultaneously considered. Caulerpa taxifolia—aquarium strain was not found at any of our survey locations. However, 14 species of Caulerpa were found at 31 of the 132 sites visited. Percent correct for our model was 61.5% for presence and 98.1% for absence. There was a positive correlation between Caulerpa spp. and seagrass beds and proximity to marinas. There was a negative correlation with latitude and human population density. The parameters in the logistic regression model assessing the association of Caulerpa occurrence with the measured variables were then used to predict current and future probabilities of Caulerpa spp. presence throughout the state. This prediction model will allow resource managers to focus their efforts in future surveys.     

Sediment biogeochemistry in an East African mangrove forest (Gazi Bay, Kenya)

The biogeochemistry of mangrove sediments was investigated in several mangrove forest communities in Gazi Bay, a coastal lagoon in Kenya, Africa. Carbon dioxide fluxes, sediment median grain sizes, sedimentary organic carbon, nitrogen and phosphorus contents and pore-water characteristics (ammonium, nitrate, sulfate and chloride) could be related to forest type. Mangrove sediments have pH values that range from 3.5 to 8.3 due to the limited buffer capacity of these sediments and intense acidifying processes such as aerobic degradation of organic matter, oxidation of reduced components, ammonium uptake by roots and root respiration. The mangrove sediments are nitrogen-rich compared to mangrove litter, as a result of microbial nitrogen retention, uptake and fixation, and import of nitrogen-rich material. It appears that mangrove sediments in Gazi Bay act as a nutrient and carbon sink rather than as a source for adjacent seagrass and reef ecosystems.     

The effects of light and nutrients on Caulerpa taxifolia and growth

Caulerpa taxifolia, an invasive species elsewhere in the world, is native to Moreton Bay where its distribution has been increasing in recent years. In Australia, dense beds of C. taxifolia are predominantly found in areas of low light and high nutrients (low water quality). Monitoring data from Moreton Bay suggests that native C. taxifolia is not directly replacing seagrass, but that there is a successional trend of seagrass loss and subsequent C. taxifolia colonization. The current study examined responses of C. taxifolia in relation to changes in environmental conditions using ambient water quality and a light/nutrient manipulative experiment. In the ambient water quality experiment we found that C. taxifolia grew significantly faster in areas with higher light (lower turbidity). The manipulative experiment demonstrated that nutrients stimulate C. taxifolia growth, however, light availability and seasonality appear to influence the response of C. taxifolia growth to nutrients in Moreton Bay. These findings suggest that C. taxifolia is unlikely to colonize seagrass beds in areas with high light and low nutrients; however, in areas with moderate light and moderate to high nutrients C. taxifolia and seagrass are likely to coexist.     

Elemental (C,N, P) and isotopic (δ13C, δ15N) signature of primary producers and their contribution to the organic matter in coastal lagoon sediment

Estuarine ecosystems are easily deteriorated by organic pollution because of its high primary productivity. To identify chemical proxies for the possible sources of autochthonous organic matter [phytoplankton-derived particulate organic material (POM), macroalgae and seagrass], we measured C:N:P and the ratios of carbon and nitrogen stable isotopes (δ¹³C and δ¹⁵N values) in two estuarine environments, the polyhaline lagoon, Lake Nakaumi, and the oligohaline lagoon, Lake Shinji, in Japan. Due to vigorous photosynthesis, the δ¹³C of phytoplankton-derived POM in Lake Nakaumi was larger than what would normally be expected from estuarine salinity gradients. Concentrations of nitrogen and phosphorus did not affect the δ¹³C of phytoplankton-derived POM. The δ¹⁵N of all plants was uniform and was higher than the δ¹⁵N of sediments. The seagrass showed a higher C:N ratio than POM and macroalgae, while the macroalgae showed a higher N:P ratio. Thus, simultaneous evaluation of C:N and N:P ratios would distinguish these three plant groups, and it would be possible to identify the source plants from the elemental ratios of the sediments.     

RNA in the degenerating silk gland of Bombyx mori

Nucleic acids in the degenerating posterior silk gland of Bombyx mori were analysed during the period from larval maturation to early pupal stage, by methylated albumin column chromatography and sucrose density gradient centrifugation. During the first half of the period, the amount of RNA decreased rapidly, but no accumulation of degradation products was detected. The ratio $\text{26}\text{S}\text{17}\text{S}$ rRNA decreased slightly. A decrease of sRNA-like polynucleotide (∼4S RNA) was faster than that of rRNA. During the latter half of the period, rRNA continued to decrease, while ∼4S RNA increased in content. This probably resulted from the degradation of rRNA. There was a significant fall in the ratio of $\text{26}\text{S}\text{17}\text{S}$, suggesting that rRNA, at least in part, was degraded by the scheme of 26 S→17 S→∼ 4S. The possibility that a part of rRNA may be released outside the tissue without complete decomposition is discussed.     

Differential anaerobic decomposition of seagrass (Zostera noltii) and macroalgal (Monostroma obscurum) biomass from Arcachon Bay (France)

Arcachon Bay is characterized by extensive meadows of the seagrass Zostera noltii. Moreover, as a consequence of eutrophication, massive proliferations of the macroalga (Monostroma obscurum) have occurred since the beginning of 1990s.This paper describes the anaerobic decomposition of biomass of both species under experimental conditions by two methods. Firstly, the dynamics of decomposition were studied in situ using litter bags. The remaining biomass and the elemental composition of the decomposing macrophytes were monitored. Secondly, degradation was studied in experimental containers under anoxic conditions in which the release of inorganic nutrients and the development of fermentative and sulfate-reducing bacterial populations were followed.The decomposition rate of total biomass was faster for macroalgae than for the vascular plants, thus corroborating previous observations. However, both in situ and laboratory experiments showed that the anaerobic decomposition of the seagrass Z. noltii resulted in rapid release of inorganic N and P, and increasing C/N and C/P ratios of the residual biomass. As a result, the recycling of inorganic nitrogen and phosphorus compounds was slightly more efficient for Z. noltii than for M. obscurum. Recycling of inorganic nutrients appears to be of a great importance to the whole ecosystem, because of the extensive spreading of Z. noltii in the bay.     

Diel differences in the seagrass fish assemblages of a Caribbean island in relation to adjacent habitat types

The fish fauna of Thalassia testudinum (König) seagrass beds was studied at two sites in the Grand Cul-de-Sac Marin Bay (Guadeloupe, French West Indies). The first seagrass bed was located near a coral reef and the second was near coastal mangroves. Both habitats were sampled during day and night, using a purse-seine and a trap net. A total of 98 species belonging to 36 families were observed. Distance-based redundancy analyses revealed two site-specific assemblages of fishes. Diel assemblage shifts were more pronounced in the seagrass beds near coral reefs than in those near mangroves, due to the existence of nocturnal trophic incursions of coral reef fishes into seagrass beds. First-order carnivores dominated the trophic structure of the fish assemblages during both day and night. At night, Haemulidae, Holocentridae and Apogonidae took the place of Labridae, Chaetodontidae and Mullidae present by day near the reef. This switch did not occur near the coast where the exchanges between seagrass beds and mangrove appear to be less important than with the reef ecosystem. Thus, it appears that the adjacent seascape habitat setting affects the intensity in diel variability of the seagrass bed fish community.     

Shedding of the major plasma membrane sialoglycoprotein from the surface of 13762 rat ascites mammary adenocarcinoma cells

ASGP-1 (ascites Sialoglycoprotein 1) the major sialoglycoprotein of 13762 rat ascites mammary adenocarcinoma cells, is shed from MAT-B1 (nonxenotransplantable) and MAT-C1 (xenotransplantable) sublines when incubated in vitro after labeling in vivo with [³H]glucosamine. The rates of shedding of label in both particulate and soluble form are similar for the two sublines, but the turnover of label in the cells is 80% greater for MAT-C1 cells ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ 2.4 days) than for MAT-B1 cells ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ 4.1 days). Shed soluble ASGP-1 was smaller than ASGP-1 in the particulate fraction by gel filtration in dodecyl sulfate. By CsCl density gradient centrifugation, gel filtration, and sucrose density gradient centrifugation, all in 4 m guanidine hydrochloride, the shed soluble ASGP-1 was found to be slightly more dense and smaller than ASGP-1 purified from membranes. No differences in sialic acid or oligosaccharides released by alkaline borohydride treatment were found between the shed soluble ASGP-1 and purified ASGP-1. These results suggest that the shed soluble ASGP-1 is released from the membrane by a proteolytic cleavage. This mechanism is supported by the inhibition of the release of soluble shed ASGP-1 by aprotinin, a protease inhibitor. Soluble ASGP-1 in ascites fluid is also smaller by gel filtration, but is more heterogeneous, suggesting a similar release mechanism in vivo followed by more extensive degradation in the ascites fluid.     

The distribution of seagrass (Zostera noltii) in the Ria Formosa lagoon system and the implications of clam farming on its conservation

Ria Formosa lagoon in southern Portugal has an important population of seagrasses that includes Zostera noltii in the intertidal area. The area is classified as a Natural Park and supports a major economic activity – clam farming. This activity has a direct influence on Z. noltii populations by removal of seagrass beds and altering habitat. Geographic Information Systems and spatial analysis were used to produce the first distribution map of Z. noltii in the Ria Formosa and to analyse interactions between clam farming and the level of seagrass protection according to the Natural Park of Ria Formosa. Stakeholder analyses, using interviews, questionnaires and participant observation, were conducted to gain a better understanding of clam farming's influence on seagrass populations. Seagrass covers 45% of the intertidal area while clam farming covers 14%. An additional 75% of the total area of Z. noltii can be potentially converted into clam cultures. The current management of clam farming is largely ineffective, representing a significant obstacle to seagrass conservation.     

The effect of nutrient additions on bacterial activity in seagrass (Posidonia oceanica) sediments

The influence of nutrient additions on benthic bacterial activity under seagrass meadows was tested by enriching five seagrass (Posidonia oceanica) meadows with nutrients over one year. We found a highly significant response of benthic bacterial activity to nutrient additions, which was reflected in greater (about two-fold) ammonification rates and, to a smaller extent, a significant tendency for a greater exoenzymatic activity. Nutrient additions significantly raised bacterial activity, without altering the seasonal changes in bacterial activity. As a result of the increased bacterial activity, the organic content of the sediments declined significantly, by about 33%, after one year of nutrient addition. Hence, nutrient additions to the seagrass meadows enhance seagrass production but also accelerate bacterial decomposition of seagrass carbon, thereby reducing the capacity of the sediments to store organic carbon. These results demonstrate that sediment nutrient availability limits bacterial activity in these Posidonia oceanica meadows, and identify bacteria as important nutrient consumers in these systems.     

Fish farming impact on decomposition of Posidonia oceanica litter

Fish farming impact on decomposition and loss of carbon, nitrogen and phosphorus fixed in seagrass litter were studied in a Posidonia oceanica meadow (Aegean Sea, Greece) using in situ incubation of senescent seagrass leaves collected under (station: cages) and away (station: control) from fish cages and deployed in a cross design of origin/station. Decomposition rate and loss of carbon and nitrogen fixed in seagrass litter were pronounced under the cages while loss of phosphorus was less evident. Decomposition was related to nutrient availability in seagrass tissue and pore water, sediment organic matter and origin of seagrass litter. When incubated under the cages, litter originated from the control decomposed faster than litter originated from the cages since the former was qualitatively better substrate for decomposers and the nutrient conditions in that station were enriched in the pore water and sediment. The lower decomposition of litter originated from cages suggests that seagrass tissues under the cages accumulate chemical deterrents, possibly in order to confront high grazing pressure, which on the other hand reduces the rate of decomposition.     

Brevetoxin persistence in sediments and seagrass epiphytes of east Florida coastal waters

A bloom of Karenia brevis Davis developed in September 2007 near Jacksonville, Florida and subsequently progressed south through east Florida coastal waters and the Atlantic Intracoastal Waterway (ICW). Maximum cell abundances exceeded 10⁶ cells L⁻¹ through October in the northern ICW between Jacksonville and the Indian River Lagoon. The bloom progressed further south during November, and terminated in December 2007 at densities of 10⁴ cells L⁻¹ in the ICW south of Jupiter Inlet, Florida. Brevetoxins were subsequently sampled in sediments and seagrass epiphytes in July and August 2008 in the ICW. Sediment brevetoxins occurred at concentrations of 11–15 ng PbTx-3 equivalents (g dry wt sediment)⁻¹ in three of five basins in the northern ICW during summer 2008. Seagrass beds occur south of the Mosquito Lagoon in the ICW. Brevetoxins were detected in six of the nine seagrass beds sampled between the Mosquito Lagoon and Jupiter Inlet at concentrations of 6–18 ng (g dry wt epiphytes)⁻¹. The highest brevetoxins concentrations were found in sediments near Patrick Air Force Base at 89 ng (g dry wt sediment)⁻¹. In general, brevetoxins occurred in either seagrass epiphytes or sediments. Blades of the resident seagrass species have a maximum life span of less than six months, so it is postulated that brevetoxins could be transferred between epibenthic communities of individual blades in seagrass beds. The occurrence of brevetoxins in east Florida coast sediments and seagrass epiphytes up to eight months after bloom termination supports observations from the Florida west coast that brevetoxins can persist in marine ecosystems in the absence of sustained blooms. Furthermore, our observations show that brevetoxins can persist in sediments where seagrass communities are absent.     

Increase in seagrass distribution at Bourgneuf Bay (France) detected by spatial remote sensing

The changes in spatial distribution of intertidal Zostera noltii seagrass beds were studied with multispectral visible-infrared remote sensing in Bourgneuf Bay (France) over a 14-year period, between 1991 and 2005. Six SPOT satellite images acquired at low tide were calibrated using in situ spectroradiometric data and processed with the Normalized Difference Vegetation Index (NDVI). A steady and linear increase in meadow areas was observed between 1991 and 2005 with total surfaces colonized by Z. noltii increasing from 208 to 586 ha, respectively. A greater increase in the densest part of the meadow (NDVI > 0.4) was also observed: it represented only 15% of total meadow surfaces in 1991 vs. 35% in 2005. The seagrass expansion took place mainly towards the lower part of the intertidal zone, while in the upper intertidal zone the meadow appeared strictly limited by the +4 m (Lowest Astronomical Tide) bathymetric level. The influence of Z. noltii above-ground biomass variations on spectral reflectance was analyzed experimentally by spectrometry. Z. noltii displays a characteristic steep slope from 700 to 900 nm, increasing with increasing biomass. A quantitative relationship obtained experimentally between NDVI and the dry weight of leaves was used to produce a biomass distribution map. The distribution of Bourgneuf Bay intertidal seagrass beds is certainly constrained by the water turbidity and we suggest that tidal flat accretion could be a significant variable explaining the observed expansion downwards. With very limited spatial interactions, oyster aquaculture cannot be considered as a threat, while a recent increase in recreational hand fishing of Manila clams within the beds could become problematic.     

Effects of the Brachyury (T) mutation on morphogenetic movement in the mouse embryo

$\text{T}\text{T}$ embryos have been first distinguishable at 8 days post coitum by their gross morphological abnormalities. By quantitative morphometry of histological sections, anomalies in the homozygotes were expressed numerically. At 8 days p.c., morphologically identifiable T-homozygotes had an increased number of ectodermal and a reduced number of mesodermal cells compared to the wild type. At 7 days p.c., embryos with a low mesoderm/ectoderm ratio were found only in litters of $\text{T}\text{+}\text{×}\text{T}\text{+}$ matings at the expected frequency. At 6 days p.c., one-fourth of the embryos in $\text{T}\text{+}\text{×}\text{T}\text{+}$ litters showed a delay in the transition from cuboidal to squamous endoderm. No such embryos were found in the +/+ × +/+ matings. In 6-, 7-, and 8-day mutant embryos, cells proliferated at statistically normal rates. Therefore, it may be said that advanced morphological irregularities of 8-day homozygotes cannot be accounted for by anomalies in cell proliferation. When the total cell number was 5 × 10⁴/embryo (8 days), a sudden change was observed in the regional distribution of mesodermal and ectodermal cells along the anteroposterior axis of $\text{T}\text{T}$ embryos. Since no regional difference in the cell cycle time was observed, these abnormalities may best be explained by anomalies in cell migration. These results strongly suggest abnormal morphology of $\text{T}\text{T}$ mutants resulting from defects in morphogenetic movement.