We examined macrophyte-environment relationships in shallow lakes located within the Prairie Parkland and Laurentian Mixed Forest provinces of Minnesota. Environmental variables included land cover within lake watersheds, and within-lake, water and sediment characteristics. CCA indicated that sediment fraction smaller than 63 μm (f<63), open water area, turbidity, and percent woodland and agricultural cover in watersheds were significant environmental variables explaining 36.6% of variation in macrophyte cover. When Province was added to the analysis as a spatial covariate, these environmental variables explained 30.8% of the variation in macrophyte cover. CCA also indicated that pH, f<63, percent woodland cover in watersheds, open water area, emergent vegetation area, and organic matter content were significant environmental variables explaining 43.5% of the variation in macrophyte biomass. When Province was added to the analysis as a spatial covariate, these environmental variables explained 39.1% of the variation in macrophyte biomass. The f<63 was the most important environmental variable explaining variation for both measures of macrophyte abundance (cover and biomass) when Province was added as a spatial covariate to the models. Percent woodland in watersheds, turbidity, open water area, and Ca+Mg explained 34.5% of the variation in macrophyte community composition. Most species showed a negative relationship with turbidity and open water area except for Potamogeton richardsonii, Stuckenia pectinata, and filamentous algae. Our study further demonstrates the extent to which macrophyte abundance and community composition are related to site- and watershed-scale variables including lake morphology, water and sediment characteristics, and percent land cover of adjacent uplands.
Approximately 1000 km2 of seagrass was lost from Hervey Bay after two major floods and a cyclone within a 3 week period in 1992. This represents 24% of the known area of seagrass along the Queensland east coast, which spans 17 degrees of latitude. Anecdotal evidence suggests that such a loss is unprecedented in the past 100 years. The seagrasses in deep water (at least 10 m depth) apparently died as a result of light deprivation caused by a persistent plume of turbid water that resulted from the floods and the resuspension of sediments caused by the cyclonic seas. Seagrasses in shallow water (less than 10 m depth) were uprooted by the heavy seas. Ten months after these events, virtually no recovery was detected. Nearly 2 years after the floods there was substantial recovery, apparently from seed germination, in deep water areas. There was virtually no recovery in shallow water areas (less than 10 m). Sediment disturbance associated with the cyclone may have deeply buried the seeds in shallow areas, or they may have died after being abraded by the churning sediment. The recovering sites in deep water were dominated by Halophila decipiens Ostenfeld, although a few sites had returned to their pre-disturbance state of a high percentage cover of tall Halophila spinulosa (R. Brown) Ascherson with a sparse understorey of Halophila ovalis (R. Brown) Hooker f. Poor catchment management and intensive penaeid shrimp trawling may have exacerbated the effects of the floods and cyclone.
Stable carbon isotope analyses of more than 140 plant, animal, and sediment samples from the Miskito Banks, Nicaragua and St. Croix, U.S. Virgin Islands, were used to assess carbon flow in Caribbean seagrass meadows and coral reefs. Plants at the base of food webs had widely divergent δ 13C values, ranging from −4.0 (Syringodium filiforme Kütz) to −34.7% (an unidentified red alga). Isotopic values of 13 of 35 algal species ranged from −8.8 to −15.0%, and were similar to the −9.9 to −13.2% values of the dominant Caribbean seagrass, Thalassia testudinum Banks ex Koenig. This isotopic similarity prevented a clear assessment of the role that Thalassia plays in supporting Caribbean food webs. However, a 4–6% 13C enrichment was found among fish from seagrass meadows and coral reefs vs. fish collected offshore. This enrichment indicated that benthic algae and seagrasses together contribute at least 48–76% of the carbon found in fish from the former, shallow-water habitats.
The genetic variability of the cosmopolitan, ubiquitous freshwater cyanobacterium Phormidium retzii was assessed using random amplified polymorphic DNA (RAPD) markers and nearly complete (ca. 95%) 16S rDNA sequences. Strains consistent with the morphological species circumscription were utilized from geographically distant locations ranging from British Columbia, Canada, in the north to Rio Claro, Costa Rica in the south, and from Rhode Island, USA in the east coast to Washington State, USA on the west coast. In addition, some strains were from geographically close (<35 km apart) locales. To assess biogeographic trends, the nine RAPD primers utilized yielded 133 distinct bands in total from the 12 strains. Non-metric multidimensional scaling (NMDS) analyses revealed that the strains isolated from Ohio, Connecticut, and Rhode Island were closely related with the Costa Rica strain and one Washington State strain associated with this group. The two strains from Canada, British Columbia and Ontario, were closely related and clustered with the two other strains from Washington State. The remaining two strains from Mexico and Missouri were loosely associated with this cluster and genetically distinct. Geographic proximity did not correlate with genetic similarity (Mantel test, r=0.235,P>0.05). A 1340 bp region of the 16S rRNA gene was sequenced for 8 of the 12 strains used in the RAPD analysis. Sequence homology among strains ranged from 88.4 to 98.4%, implying the presence of cryptic species within this group of strains. Given the lack of sequence similarity, P. retzii as presently circumscribed most likely represents several cryptic species not clearly distinguishable with light microscopy of morphological features. This conclusion may explain the lack of correlation between geographic proximity and genetic similarity. This research provides further strength for the case of species of cyanobacteria being more localized rather than having global distributions.
Vegetation changes in the Yahara lakes around Madison, Wisconsin, USA, are evaluated using historical data. Present vegetation is less diverse and less extensive, produces less biomass and is composed of more disturbance tolerant species than it was 80–100 years ago. Many changes are related to human impacts that began about 150 years ago which directly altered plant habitat, decreased water clarity, were toxic to plants, removed plant biomass or reproductive structures, or resulted from exotic invaders. The impacts are interrelated and overlap in time and space so change did not occur from simple cause and effect relationships. However, cumulative effects of the impacts are very evident. Management recommendations are made that are applicable to North American lakes with a similar history.
Species separation in charophytes is primarily based on vegetative morphology, but the taxonomic delineation at species level worldwide is inconsistent. Here we compare ecophysiological characteristics of selected Chara species with vegetative morphological traits and genetic data. Four populations of the Chara baltica–Chara intermedia species cluster from locations along a north-south gradient through Europe were investigated. Physiological differences indicate habitat-specific adaptations. The Baltic Sea population from Hiddensee was found to be separated from a freshwater river population close to Munich with respect to light and salinity acclimation capabilities. However, a population from a brackish Mediterranean pool near Montpellier and one from the continental brackish water lake “Salziger See” near Halle, Germany, show intermediate ecophysiological characteristics, suggesting a continuum. Genetic analyses using AFLP (amplified fragment length polymorphism) match the results of the physiological analyses. The individuals of the Mediterranean and the Salziger See neither cluster to the typical C. baltica from the Baltic Sea nor to the C. intermedia from the freshwater habitat, but instead take an intermediate position. These results are stable against the background of a larger AFLP dataset on charophytes. Morphological analysis revealed no distinct groups and we therefore conclude that the C. baltica–C. intermedia cluster forms a physiological, morphological and genetic continuum.
Intensification of the maintenance of drainage channels on the Monmouthshire Levels, notably increased dredging activity and the introduction of herbicide sprays, are considered to have improverished the aquatic macrophyte flora of the drainage channels. A reconstruction of the flora of the channels for the past 150 years, mainly using published plant records and herbarium specimens, has shown that 55 of the 100 species known to have occurred in the channels have remained either widespread or restricted in their distribution. Six species have apparently disappeared from the channels during this time, 11 species have recently appeared, whilst others have changed their spatial distributions. Neither dredging nor herbicide sprays have had obvious long term effects on the flora though dredging is important in initiating a hydroseral succession in the channels. Other important factors which cause changes in distributions are salinity fluctuations, shading, species introduction by man and, possibly, climatic changes.
A comparison is made of the wasting disease that struck the whole Atlantic population of Zostera marina L. in the 1930s and a current outbreak of a rather similar disease in Z. marina beds along the north-eastern coasts of the U.S.A. Although the disease phenomena on the plants appear to be very similar, disease-related declines of Z. marina are at present still very local. In Europe, diseased plants have been found, but no declines have been observed.The wasting disease in the 1930s was not investigated before the epidemic reached a devastating stage. Present observations may indicate that a new widespread die-off may be developing. In order to facilitate the study of the current epidemic, a scenario of disease and related decline, with several variants, has been elaborated, based on the existing knowledge of the epidemic of the 1930s, but also clearly showing the gaps in this knowledge.
The decline of reed populations along the shores of the River Havel in Berlin is traced on the basis of regular population surveys. The causes for the observed reduction are described, as well as the effects on the biocommunities and the environment at the water's edge. Finally, the protective measures carried out are assessed.
Changes in seagrass coverage in Cockburn Sound from 1967 to 1999 were assessed from aerial photographs using modern mapping methods with the aim of accurately determining the magnitude of change in hectares of seagrasses between 1967 and 1999 and to set up a baseline for future monitoring of seagrass loss in Cockburn Sound. Firstly, coverage and assemblages of seagrasses in Cockburn Sound were mapped using the best available aerial photographs from 1999, rectified to a common geodesic base with comprehensive groundtruth information, and with a semi-automated mapping algorithm. Then the same technique was used to map historical seagrass coverage in Cockburn Sound from aerial photographs taken in 1967, 1972, 1981 and 1994.The seagrass coverage in Cockburn Sound has declined by 77% since 1967. Between 1967 and 1972, 1587 ha of seagrass, were lost from Cockburn Sound, mostly from shallow subtidal banks on the eastern and southern shores. By 1981, a further 602 ha had been lost. Since 1981, further seagrass losses (79 ha) have been restricted to a shallowing of the depth limit of seagrasses, localised losses associated with port maintenance and a sea urchin outbreak on inshore northern Garden Island. There has been no recovery of seagrasses on the eastern shelf of Cockburn Sound after nutrient loads were reduced in the 1980s, suggesting that this shallow shelf environment has been altered to an environment not suited for large-scale recolonisation by Posidonia species.
Reed (Phragmites australis (Cav.) Trin. ex Steudel) is one of the dominant plant species in European land-water ecotones. During the past decades reed belts have died back, especially in central and eastern Europe. The aim of the European Research Programme on Reed Die-back and Progression (EUREED), was to examine how increasing eutrophication, changed water table management, temperature, reduced genetic variation and their interactions may contribute to reed die-back. Eutrophication appeared to be a key factor, but the effects on P. australis were indirect, via the accumulation and decay of litter and allogenous organic matter, rather than acting directly via disturbed carbohydrate cycling or reduced porosity of the aerenchymous plant tissue. The formation of toxic byproducts of decomposing litter in anoxic environment, such as acetic acid, may reduce reed vigour. Sulphide may act as a principal toxin especially at brackish sites, such as Lake Fert, Hungary. There were large differences in genetic variability. However, populations from eutrophic sites did not grow faster and were not more plastic than populations from oligotrophic sites. Variation within populations could be large as compared to variation between populations when exposed to nitrogen, liquid manure and litter. DNA-fingerprinting showed differences between land and water reeds. This may be due to a differential selective force during establishment, e.g., when the water table recovers after a drawdown. Selection by water table during establishment could affect the susceptibility of clones in the mature stage for indirect effects of eutrophication. It was concluded that in stagnant water bodies the present water table management enforces the effects of eutrophication on the reed die-back. In addition, local disturbances, such as the mechanical mowing of reeds may enhance reed die-back.
Gibberellic acid (GA3) and chlorflurenol (methyl-2-chloro-9-hydroxyfluorene-(9)-carboxylate) markedly enhance the effect of 2,4-D on water hyacinth under greenhouse conditions. Plants which were sprayed with 100 g ha−1 of 2,4-D in combination with either 6 g ha−1 of GA3 or 45 g ha−1 of chlorflurenol died within a few weeks. Application of 100 g ha−1 of 2,4-D alone was not effective while 1000 g ha−1 killed the plants within a week. As a consequence it might be expected that the effective 2,4-D concentration for controlling water hyacinths in the field can be decreased if this herbicide is applied in combination with GA3 or chlorflurenol. This might lower the risks of 2,4-D causing damage to nearby crops or vegetations.
A study was conducted to determine the effect of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) on nine submersed macrophyte species. The first objective of the study was to investigate the sensitivity of various endpoints in macrophyte toxicity tests. A second objective was to investigate the implications of hormesis in the risk assessment of 2,4-D. 2,4-D was applied in concentrations ranging from 10 to 3000 μg L−1. Endpoints determined 4 weeks after the start of the treatment were based on shoot and root growth in water. The EC50s were calculated using models excluding and including a parameter describing hormesis. Results indicated that the total length of the roots can be regarded as a sensitive endpoint for the response of a macrophyte to 2,4-D. For the tested rooted macrophyte species, the EC50 values for the length and number of the roots ranged from 92 to 997 and from 112 to 1807 μg L−1, respectively. At low concentrations (10 and 30 μg L−1), stimulation of some of the endpoints (hormesis) was found for several of the species. Although hormesis may have ecological implications, its importance for the ecological risk assessment of 2,4-D in this study was limited.
Numbered in hundreds of thousands, and ranging from <1 ha in area to Europe’s largest lake (Lake Ladoga), North European softwater lakes (lime-deficient waterbodies, ranging from low-nutrient, low pH to meso-eutrophic, circumneutral pH) support an important plant component of the biodiversity resources of Europe. Within this region (comprising the British Isles, Scandinavia, and the North European Plain from Brittany to the Baltic States), some 24 softwater lake euhydrophyte community types occur, including stress-tolerant isoetid communities which particularly characterise softwater lake vegetation. A number of rare species are supported, including plants, such as Eriocaulon aquaticum, which are found nowhere else in Europe. In line with the predictions of the hump-back model of diversity–productivity relationships in lakes, the individual plant diversity found in softwater lakes in this region is low (six or fewer euhydrophyte species present, e.g. in acidic, ultraoligotrophic lakes) to moderately high (>20 euhydrophyte species in lakes with, for example, mesotrophic conditions with pH <7.0). Overall, however, a large set of plant species (>100) occurs, forming assemblages which play a vital role in the functioning of softwater lake ecosystems. Lake area, altitude, trophic state and water quality have been found to be good predictors of macrophyte species richness for lakes within this area. Major threats to the survival of softwater lake vegetation include acidification, eutrophication, increased recreational use of lakes, and the effects of lake regulation for hydro-electric schemes. All such pressures tend to reduce the diversity of typical softwater species present in affected lakes. In addition, there is increasing concern about the possible impacts of global CO2 increase for the continued survival of (especially) isoetid vegetation which is strongly adapted to ambient conditions of limited dissolved inorganic carbon availability in softwater lakes. The implications of such global change for the maintenance of softwater lake plant diversity in northern Europe are potentially severe.
Analysis of field survey data collected during 1999, from waterbodies (river channel, secondary channels and backwaters, temporary lagoons, permanent lagoons) within the riverine floodplain wetlands of the Upper Rio Paraná in southern Brazil showed: (i) strong dominance by five aquatic macrophyte species (most important being Eichhornia azurea), out of 28 species recorded at the survey stations; (ii) based on TWINSPAN clustering, the existence of three major aquatic vegetation types. These comprised an E. azurea–Limnobium laevigatum sub-community (at 20% of stations sampled); an E. azurea–Salvinia sub-community (commonest, at 67% of stations); plus a Nymphaea amazonum–Myriophyllum aquaticum community (rarest, at 13%) which occurred in habitats showing significant between-type differences in environmental conditions for water (depth, underwater light regime, conductivity, pH, alkalinity, phosphate) and sediments (redox potential, calcium, total Kjeldahl nitrogen, phosphate, iron); (iii) predictable patterns of variation in plant size and shape, among populations of the dominant plant species present: E. azurea, Cyperus diffusus, Polygonum ferrugineum, Salvinia spp., Eichhornia crassipes, N. amazonum, along gradients of water and sediment physico-chemistry;(iv)significant variation in both macrophyte community abundance and species diversity between the three vegetation types, with the highest species richness occurring in the E. azurea–L. laevigatum sub-community, but highest community biomass in the E. azurea–Salvinia sub-community.Minimal linear models are presented which use a combination of environmental and vegetation attribute data as predictor variables to predict species richness and biomass of aquatic vegetation for the floodplain waterbodies. Such models can be used to assess vegetation response to likely scenarios of future change in river conditions in sub-tropical areas, for example modification of flow regime by dam construction.
The maximum colonisation depth (MCD) of macrophytes in lakes is considered a robust ecological indicator of system health. The accuracy of five boat-based survey methods (double-headed rake, Ekman grab, bathyscope, hydroacoustics, and underwater photography) was evaluated along depth-transects within a shallow eutrophic loch (Loch Leven, Scotland, UK) in 2006. Data from an earlier field campaign conducted in 1993 (33 transects) were used to assess sample frequency and error associated with the use of the double-headed rake method. The double-headed rake consistently sampled more species than the Ekman grab or bathyscope, probably as a result of its ability to sample relatively larger areas of the lake bed in comparison to the other methods. The bathyscope method underestimated the MCD in comparison to all other methods. This appeared to be because the turbidity of the water-column restricted the depth of view when using the bathyscope. The double-headed rake and underwater photography methods were in good agreement, with maximum observed MCD estimates of 5.0 m and 4.5 m water depth, respectively (2006). Using the 1993 data (MCD observations taken from 33 selected transects), we estimated that over 5 transects were required to ensure a MCD estimate greater than the 75th percentile from the distribution of all 33 colonisation depths, more than 15 replicate transects were required to ensure an estimate greater than the 90th percentile, and more than 20 replicate transects were required to ensure an estimate greater than the 95th percentile.
A 6 year (1986–1991) study in Water Conservation Area 2A (WCA2A), a remnant portion of the Florida Everglades, was conducted in which multiple plots within six study sites across a surface water nutrient gradient were monitored for water quality, depth, and sawgrass (Cladium jamaicense Crantz) and cattail (Typha domingensis Pers.) densities. A negative logarithmic relationship was demonstrated between total P concentrations and distance from the nutrient source (levee L-39). Dense cattail stands illustrated by remote sensing in 1987 coincided spatially with high sediment total P concentrations measured in 1990. Cattail density appeared to increase more rapidly than did sawgrass densities during wet years. Cattail decline was greater at nutrient enriched sites than at the low nutrient site during dry years. Drought stimulated sawgrass expansion at nutrient enriched sites, but not at the low nutrient site. Cattail increased at both nutrient enriched and low nutrient sites following fire, yet fire and nutrient enrichment together appeared to stimulate greater increases in cattail density. Multi- and univariate ANOVA revealed significant nutrient effects on plant density during the last 3 years of the study (P<0.05). Stepwise regression analysis suggested that total P loading into WCA2A best explained plant density fluctuations at three of the four sites closest to the levee, while hydrology best explained plant density fluctuations at the site most distant from the levee. Although the single low nutrient site provided an inadequate basis for understanding sawgrass and cattail dynamics under low nutrient conditions, the results of this study support two hypotheses of causal factors leading to the spread of cattail in WCA2A: (1) nutrient enrichment stimulates cattail growth; (2) a prolonged hydroperiod is conducive to cattail proliferation.
Microcosms consisting of a sublittoral seagrass meadow from Botany Bay, N.S.W. were maintained in aquaria in a controlled environment room from March to August 1979. Oxygen production and consumption in the microcosms and leaf turnover of the seagrass, Posidonia australis Hook.f. were measured before, during and after a 7-day treatment in June with Bass Strait crude oil and the dispersant Corexit 8667. Four microcosms received oil and two of these received dispersant.The leaf turnover of P. australis was not significantly affected by the addition of oil and dispersant. Photosynthetic oxygen production decreased and respiration incresed in the microcosms during treatment. In August, 40 days after treatment, oxygen production rates and P/R ratios in the oil-treated microcosms were higher thatn rates measured before treatment. The oil-and-dispersant-treated microcosms did not show this trend. The results of this study indicate that a more severe stress is placed on the P. australis dominated benthic community by oil and dispersant than by oil alone.
Salinities greater than 35 ppt and completely submerged conditions reduced growth of Sarcocornia perennis (Mill.) A.J. Scott, an important intertidal salt marsh macrophyte occurring in a number of South African estuaries. Highest mortality of S. perennis plants occurred with the submerged treatments. Plants grew best under water-saturated soil conditions at salinity values between 0 and 15 ppt.Sarcocornia perennis plants submerged initially at approximately 5 cm below the water surface showed rapid stem elongation above the surface. Succulent leaves below the water decomposed and this was more rapid in low salinity treatments. Inundated conditions will also prevent flowering and seed production, thus reducing the potential for recruitment of new plants when water levels eventually subside.
Pond B is an 87-ha abandoned reactor cooling reservoir on the Savannah River Plant, South Carolina. Since abandonment in 1964, the reservoir has reverted to a low conductivity and slightly acidic water chemistry, and has also been colonized by abundant macrophyte vegetation. To determine dominant species and their depth distributions, and to estimate seasonal changes in standing crop, above-sediment biomass of macrophyte vegetation was sampled at four water depths three times during the 1986 growing season. Three floating-leaved species, Nymphaea odorata Ait., Brasenia schreberi Gmel and Nymphoides cordata (Ell.) Fern., comprised 51.8% of biomass harvested, with greatest biomass at 1.5 m and in the October sampling. Free-floating Utricularia spp. (mostly U. floridana Nash) contributed 32% of harvest biomass, were abundant at all sampling depths and had greatest standing crop in May. Submersed rooted species comprised 15.4% of biomass harvest, dominated by Cobomba caroliniana var. pulcherrima Harper in deeper portions of the littoral zone. The species composition found in Pond B was similar to that of aquatic plant communities in other softwater Coastal Plain habitats (such as Carolina Bay ponds and rain-fed swamps), and differed from species composition reported for Par Pond, a neighboring reservoir with higher conductivity. These comparison suggested that differences in water chemistry may have been important in determining establishment of macrophytes species in these two reservoirs.
The regeneration (regrowth into viable plants) and colonisation (establishment in the sediment) abilities of vegetative fragments of six aquatic plant species (Elodea canadensis Michaux, Hippuris vulgaris L., Luronium natans (L.) Rafin., Potamogeton pusillus L., Ranunculus trichophyllus Chaix, Sparganium emersum Rehm.) occurring in habitats frequently disturbed by floods along the Rhône River (France) were compared through a laboratory experiment. Six types of plant fragments were collected from the plants and placed in water over sediment. Their development and/or their rooting into the sediment were recorded over 10 weeks. The species exhibited different survival tactics to survive after fragmentation: (1) fragments from Sparganium and Ranunculus developed roots and rapidly established into the sediment whereas (2) fragments from Luronium, Hippuris and Elodea developed many propagules that could be dispersed but did not establish within the 10 weeks of the experiment. The fragments of P. pusillus never established nor regenerated. The results showed a trade-off between regeneration and colonisation related to the patterns of recolonisation of cleared patches of the species and their ecological requirements. Maintenance of these species in ecosystems frequently disturbed by floods can partly be explained by their high regeneration abilities, and results are discussed in terms of life-history traits and ecological strategies.
Stand structure and biomass production of Phragmites australis (Cav.) Trin. ex Steud. were analyzed along north–south and east–west transects in the Burullus coastal lagoon (N Egypt, 410 km2) at monthly intervals over a period of 1 year (February 2003 until January 2004). For this purpose, young and old stands were selected at eight different locations in the lagoon. It was found that the north–south transect mainly represented a fertility gradient (207–286 mg l−1 TN, 30–106 mg l−1 TP), while the east–west transect was associated with significantly decreasing salinity (7–4 ppt). All morphological and biomass variables of P. australis were significantly different between young and old stands. On average, the old (7.3 ± 0.2 kg DW m−2) accumulated three times more total above-ground biomass than the young stands (2.5 ± 0.1 kg DW m−2). Shoot height, diameter and shoot dry weight significantly increased by 25–50% with increasing fertility along the north–south transect. Shoot density significantly decreased from north to south, while it almost doubled in the north sites from 109 ± 6 to 216 ± 7 shoots m−2 along the west–east transect. In separate stepwise multiple regressions, variation in water quality explained 34–63% of the variation in morphology and total above-ground biomass in the old stands (salinity and water level were most important for biomass, transparency also for height and density) while it explained 16–42% of variation in young stands (mainly transparency).
Phosphorus (P) demand in extremely oligotrophic shallow water wetlands often exceeds supply and maintains water chemistry in a P-limiting condition. Phosphorus uptake by the calcareous periphyton community in the oligotrophic Florida Everglades was examined. Phosphorus removal from solution was used to obtain uptake parameters for epipelon, epiphyton, and metaphyton. Nutrient ratios (C:N:P) were higher in epiphyton compared to metaphyton or epipelon but all periphyton types were P-limited. Michaelis–Menten kinetic experiments resulted in Km values ranging from 8.5 to 16.4 μM and Vmax values ranging from 0.24 to 0.74 μmol g−1 dry weight min−1, in the order epiphyton, metaphyton, epipelon. Removal of inorganic P (Pi) as KH2PO4 and dissolved organic P (Po) as ATP from solution was best described by a first-order equation with rate constants ranging between 0.02 and 0.17 min−1. The values of Km were greater than ambient dissolved reactive P (DRP) concentrations (<0.2 μM); therefore Vmax is not reached under normal field conditions. The hydrolysis of Po as ATP was rapid, being >0.67 μmol g−1 DW min−1. After hydrolysis Po uptake was similar to Pi uptake. It was hypothesized that P adsorption with CaCO3 in the periphytic matrix would add to the removal of P from solution. Using and ATP and a 0.01 M HCl extraction technique, P incorporated by epipelon was partitioned into biotic and abiotic compartments. The biotic compartment contained >83% of the incorporated P after 12 h incubations. Biological demand exceeds abiotic adsorption in this P-limited system but adsorption mechanisms are responsible for a portion (<15%) of water column P removal.
We assessed natural rates of floral abortion in four common mangrove species from northern Australia and subsequently manipulated pollination experimentally. Sonneratia alba J. Smith exhibited the highest rate of fruit set of the four species (23%), indicating this mangrove was best able to utilise the natural pollination opportunities provided. Fruit set in S. alba appeared, however, to be pollinator limited, as large increases in fruit set occurred after manual cross-pollination of flowers. Avicennia marina (Forsk.) Vierh. had the highest rate of natural pollination, but fruit set was lower (15%) and appeared to be impeded by resource limitations. Although a range of insects visited Ceriops australis (C.T. White) Ballment, T.J. Sm & Stoddart, the rate of fruit set was low (3%) and the capacity for flower fertilisation limited, despite evidence of autogamy in this species. There was an indication of both resource and pollinator limitation in C. australis. Rhizophora stylosa Griff. exhibited limited fruit set (0.5%), possibly due to limiting maternal resources and the lack of adaptation of flowers to either animal or wind pollination. Large increases in fruit set were recorded after manual cross-pollination of R. stylosa flowers. R. stylosa and C. australis, characterised by resource rich propagules with long periods of development, both aborted a large proportion of propagules during the fruit maturation process.
The morphology of Potamogeton pectinatus L. varies markedly between two sites, with different degrees of wave exposure, in the northern Baltic proper. To study the morphological variation attributable to phenotypic plasticity, tubers and sediment were reciprocally transplanted between the two sites.Several plant characters respond plastically to sediment type. The average values in muddy compared with sandy sediment was: for plant dry weight 1.0 g and 0.4 g; for total rhizome length 1.3 m and 0.6 m; for root:shoot ratio 0.08 and 0.21 respectively. Plants collected at the sheltered site were significantly more phenotypically plastic in total growth and rhizome length, than plants originating from the exposed site. The size of the transplanted tubers had a significant impact on growth and survival of the plants. Shoot length and branching were greatly affected by the above-sediment conditions, i.e. mainly wave exposure. The shoots were on average 21 cm tall at the sheltered site and only 10 cm at the wave exposed site. This was mainly due to a plastic response in internode length and to a minor extent to dislodgement of shoot parts. Many of the observed plastic responses in morphology of P. pectinatus result from phenotypic plasticity in growth. The plasticity in the root:shoot ratio to the nutrient content of the sediment and the plasticity in growth pattern to the degree of wave exposure are however suitable adjustments in resource allocation to the environmental conditions. All measured characters showed significant phenotypic plasticity in the experiment, nevertheless large morphological differences remained between plants from the two sites, when grown under the same environmental conditions.
Annual (1990–91) mean values of total biomass of Thalassia testudinum (Banks ex König) at three stations in different environments in a Caribbean reef lagoon varied between 573 g dry wt m−2 (±167 SD) in a back-reef station, 774 g dry wt m−2 (±167 SD) in a coastal fringe and 811 g dry wt m−2 (±131 SD) in a mid-lagoon station. Corresponding ratios of mean above-ground to total plant biomass were 4.8% (±1.0 SD), 6.7% (±1.6 SD) and 8.6 (±2.0 SD) respectively. Production of above- and below-ground plant parts were calculated using the annual mean formation rates for leaves. The calculated annual mean total plant production (excluding root production) was 426, 1015 and 1116 g dry wt m−2 at the back-reef, coastal and mid-lagoon station respectively. Above-ground production accounted for 60.8%, 60.3% and 65.9% of the total production at these respective stations. Unimodal seasonal variations in blade length synchronized with earlier reported fluctuations in blade growth, whereas variations in number of blades per shoot reflected seasonal variations in blade initiation rates.
Peak above-ground biomass of six annually mown salt-marsh angiosperm communities was measured over a 13-year period. Top soil salinity and soil moisture content in the six communities were measured during two dry years. Over the 13-year period the communities showed a synchronous pattern in peak biomass. Year-to-year variation in this biomass could be explained by the rainfall deficit during the growing season, while inundation frequency did not contribute to the regression model. Soil salinity and soil moisture content were significantly correlated with the rainfall deficit over a 2-year study period. Climatically induced year-to-year variation in soil conditions was held responsible for the observed year-to-year variation in peak above-ground biomass.
Spatial and temporal variations in net above-ground primary production (NPP) and litter turnover rate were studied, from 1987 to 1993, in a mangrove forest bordering Laguna de Terminos, Mexico. NPP, the sum of total litter fall and wood production, was measured over the entire study period in three zones in a basin forest: zone I, where Rhizophora mangle (red mangrove) occurs but Avicennia germinans (black mangrove) is the dominant species; zone II, a scrub forest of A. germinans; zone III, where larger A. germinans trees occur. In 1991, a fringe zone dominated by A. germinans and R. mangle was added to the study. Three distinctive climatic seasons occur in the region: rainy, dry, and cold front (locally named ‘nortes’). Average total litter fall in the fringe zone (793 g m−2 year−1) was significantly higher than in the basin forest (496, 307, and 410 g m−2 year−1 for basin zones I, II, and III, respectively). All zones showed significant differences among seasons with the norte season having significantly lower litter fall. Litter turnover rates were about 7 months in zones I and II and 10 months in zone III, reflecting the low tidal range that occurs in the basin forest. Low litter turnover rates in the basin forest were reflected in a high organic matter standing crop. Annual average stem growth was significantly higher in zones I and III (1.27 and 1.36 kg per tree year−1, respectively) than in zone II (0.62 kg per tree year−1). Above-ground NPP rates in the basin forest (399–695 g m−2 year−1) were lower than in fringe and riverine forests, reflecting patterns of litter fall and wood production. There was no seasonal variation in soil salinity but the basin forest had significantly higher soil salinity than the fringe forest. Spatially, mean soil salinity was inversely related to litter fall. Long-term patterns in soil salinity, precipitation and air temperature explained 74% of the inter-annual litter fall variability. Over the 7 year study, productivity in zone II was more variable than in zones I and III, and productivity (litter fall and wood growth) were less variable than litter standing crop and turnover.
To reduce time and labor in the field, digital photographs were used to assess quantities of live and dead plant biomass in the Florida Everglades. Profile-oriented images were acquired of vegetation contained within an open-sided frame having a black backdrop of known area. Using imaging software, the pictures were transformed so that live, dead, and the absence of plant material were represented by the colors green, red, and black, respectively. Subsequent pixel counts within the boundaries of the backdrop, combined with average stand height values, gave estimate terms that were plotted against the dry weights of all aboveground materials harvested from within the frame. Regression analyses showed strong correlations (R2 > 0.84) between estimated and actual dry weights, indicating that this technique is a rapid and accurate way to assess biomass in monospecific stands of these plants.
Aboveground production of macrophytes in tidal marshes was studied for 3 years at Montmagny and Cap St. Ignace on the south shore of the St. Lawrence River in Québec, Canada. Year-to-year variation of production of the dominant species, Scirpus americanus Pers., was observed and related to variation in weather during the growing season. With the exception of Zizania aquatica L., relative production of the macrophytes for different parts of the marshes remained constant in successive years. A multiple regression procedure was used to establish the relationship between production and a series of independent variables including biotic (goose use and plant competition) and abiotic factors (submersion, substrate hardness, soil texture and sedimentation). A lower percentage of organic matter in the soil and a firm substrate were associated with a greater production of S. americanus. Accretion of sediments was positively correlated with the production of S. americanus and Sagittaria spp., whereas the percentage of time of submersion was negatively related with production of Sagittaria spp., Eleocharis spp. and Scirpus torreyi Olney. Competitive interference was apparent between S. americanus and S. torreyi and between Eleocharis spp. and Zizania spp. No measured abiotic characteristic of the marsh explained the variation of Zizania production.
Aboveground and belowground biomass relationships of 15 annual and perennial freshwater tidal wetland macrophytes were examined. The data showed that regression equations may be used with confidence to estimate belowground biomass from aboveground biomass for most species. The linear regression model was suitable except for one species which had a large belowground component and for which the exponential model was more appropriate. Belowground: aboveground biomass ratios were significantly different for the 8 annual species examined. At peak biomass, all annuals allocated less than one third of the total net annual production into belowground structures. They exhibited distinct seasonal patterns of biomass allocation with more biomass incorporated into belowground components during the early part of the growing season. Perennial species exhibited 4 patterns of biomass allocation with Peltandra virginica (L.) Kunth having a significantly greater mean belowground: aboveground biomass ratio than other perennials. Factors that may control biomass allocation patterns include depth of rooting and life history strategies.
Chemical compounds isolated from Chara have been shown to inhibit algal photosynthesis in laboratory experiments, supporting the concept of negative allelopathy in Chara. However, evidence for this phenomenon in situ has not been obtained.Studies of the relationships between lake phosphorus levels and phytoplankton biomass in six Chara lakes demonstrated that these lakes have a phytoplankton biomass of predictable size. This indicates that Chara does not release allelopathic compounds in situ. The reported densities of epiphytes on these plants also indicate the absence of allelopathy by Chara in situ.
Since Antarctic plants experience a wide range of ultraviolet (UV) exposure, the pigment content of Antarctic algae (Palmaria decipiens (Reinsch) Ricker, Enteromorpha bulbosa (Suhr) Montagne, Prasiola crispa (Lightf.) Kütz. sp. antarctica (Kützing) Knebel) was monitored over a year. In summer the mature form of the marine rhodophyte Palmaria contains a range of UV-absorbing pigments in high concentration providing a broad absorbance with a maximum at 337 nm. Juvenile fronds develop through winter with smaller absorbance maxima at 322 nm, 309 nm and 295 nm. The terrestrial chlorophyte Prasiola crispa contains a single UV-absorbing pigment with a maximum at 325 nm. Compared with other green algae, including the marine Enteromorpha, the UV-absorbing pigment in Prasiola is present in high concentrations. Variations in the level of UV-absorbance relative to chlorophyll in Prasiola, appear to correspond with varying UV exposure. To test this, Prasiola was maintained with an enhanced ratio of UV-B to visible light to simulate the effects of stratospheric ozone depletion. After 4 weeks the chlorophyll content and photosynthetic rates were reduced in the presence of enhanced UV-B light, but the ratio of UV-absorbing pigments to chlorophyll was unchanged. This suggests that even for Antarctic algae, that contain high levels of UV-absorbing pigments, exposure to sunlight with an increased ratio of UV-B to visible light is stressful.
The occurrence of mycosporine-like amino acids (MAAs) in freshwater and terrestrial microalgae and their accumulation in response to UV-B exposure were examined. Three UV-B-resistant species, Enallax coelastroides,Scenedesmus sp. and Scotiella chlorelloidea, and four UV-B-sensitive microalgal species: Ankistrodesmus spiralis, Chlorella minutissima, Chlorella sorokiniana and Pseudococcomyxa sp. were found to contain MAAs. However, Coelastrum microporum did not appear to contain any MAA. A total of seven MAAs was identified among the tested species. Besides five known MAAs: mycosporine-Gly (λmax = 310 nm), palythine (λmax = 320 nm), asterina (λmax = 330 nm), shinorine (λmax = 334 nm) and porphyra (λmax = 334 nm), two novel compounds (compound I: λmax = 302 nm, Rt = 2.6 min; compound II: λmax = 292 nm, Rt = 7.3 min) were found in Ch.sorokiniana and Scenedesmus sp. The accumulation of MAAs was induced by both artificial and solar UV-B radiation, which was observed in both the UV-B-resistant and UV-B-sensitive species. No clear correlation between MAA accumulation and UV-B resistance of the photosynthetic apparatus was established. The response of MAA accumulation to UV-B exposure was species-dependent and individual MAA-specific.
Electron transport rate (ETR) calculations require values of irradiance, effective quantum yield of chlorophyll fluorescence (ΔF/F′m), the distribution of energy between PSII and PSI and leaf-specific photosynthetic absorptance. We conducted virtually simultaneous replicated measurements of diel changes in ΔF/F′m of Posidonia australis in situ, and determined leaf-specific photosynthetic absorptance in vivo using integrating spheres. Leaf-specific non-photosynthetic and photosynthetic absorptance, corrected for spectral reflectance, comprised 16 and 68% of total spectral absorptance (84%); about 7% of photosynthetically active radiation incident on the leaves was reflected. Virtually simultaneous quantum-yield measurements of eight short shoots provided an estimate of among-sample variability in ΔF/F′m: generally the standard deviation was ∼10% of the mean except for a few hours around midday when the variability rose to ∼25% of the mean. Our results indicate that leaf-specific non-photosynthetic absorptance, spectral reflectance and shoot-to-shoot variability need to be taken into account when estimating ETR in the field.
Increasing levels of FeEDTA to the nutrient solution in which Hydrilla verticillata (L.f.) Royle was growing resulted in changes in the overall growth and development of the plants. Tissue levels of Fe and chlorophyll increased as Fe levels in the nutrient solution were increased from 0 to 8.0 p.p.m. Hydrilla absorbed and accumulated Fe in much larger amounts than were required for its optimum growth. The highest tissue Fe concentration was obtained with 6.0 p.p.m. Fe in the growing medium. However, the best growth was obtained at 8 p.p.m. Fe. Increasing levels of Fe in the nutrient solution resulted in progressive decrease in Mn uptake by the plants. Under the conditions of this study, best growth and greatest dry weight of Hydrilla were obtained at an exceptionally high Fe/Mn ratio (85).
Bio-optical characteristics and photosynthetic performance of five strains of benthic diatoms belonging to three different species (Nitzschia thermalis Kützing var. minor Hilse sensu Podzorski, Nitzschia laevis Hust and Navicula incerta Grunow) were examined. The effects of treatments with monochromatic blue light (BL) were also determined. The values of photosynthesis rate at saturating photon fluence rate (PFR) (Pmax) and ascending initial slope of the P-PFR curves (α) ranged from 0.22 to 3.58 μmol O2 per mg Chl a min−1 and from 0.012 to 0.169 μmol O2 per mg Chl a (μmol photons m−2 s−1)−1 respectively. Onset of light saturation parameter (Ek) ranged from 43 to 12 μmol photons m−2 s−1. These values were comparable to those reported from measurements performed with sub-tidal benthic communities. In vivo effective optical cross-section of Chl a (a∗ values) was correlated to average cell surface area. Interestingly, a∗ and α were also negatively correlated, probably indicating that a higher pigment packaging affects negatively to α. The acclimation of the five strains to monochromatic BL for 2 weeks produced a reduction of Pmax and α although the effective quantum yield of photosystem II (ΔF/Fm′) was not affected. In addition to these changes, chlorophyll a increased and a∗ decreased in the five strains. Therefore, it was concluded that the reduction of a∗ could contribute to the changes in the photosynthetic performance in BL.
Bacteria growing on and in close association with the rhizome detritus of two seagrasses, Zostera marina L. and Thalassia testudinum Banks ex König, were examined using epifluorescence and scanning electron microscopy. The microbial community consisted of a diverse assemblage of bacteria dominated in biomass by large rod-shaped and filamentous cells. The large size of cells and the occurrence of measurable acetylene reduction activity suggested that a healthy, growing population of bacteria was associated with the rhizome detritus. Bacteria carbon biomass ranged betwee 5.2×10−5 and 1.7×10−3 g C gdw−1 of rhizome detritus. Depending on cell doubling times, bacterial metabolism could account for a substantial portion of the turnover of rhizome detritus. Estimates of potential microbial production, nitrogen fixation and the physico-chemical nature of rhizome detritus are discussed and we propose hypotheses for the disposition of this detrital organic matter.
New remote sensing technologies have emerged to quantitatively assess submersed aquatic vegetation abundance and distribution. We evaluated a hydroacoustics global positioning system to map the percent of the water column occupied by submersed vegetation (referred to here as biovolume) in three Minnesota (USA) lakes. We evaluated the relative accuracy and precision of digital biovolume maps produced by three interpolation methods (inverse distance weighted (IDW), kriging and spline) after using a non-parametric regression smoother to remove a non-linear depth trend. Interpolated predictions with all methods were relatively accurate in all lakes; however, precision varied among lakes. In all cases, kriging interpolation produced the best predictions when compared with observations in independent verification data sets. However, IDW predictions were only slightly less precise. Map detail was lost when sampling effort was reduced from 10 m transect spacing to 20 or 40 m, although estimates of littoral-wide means did not change appreciably. We concluded that hydroacoustics combined with geostatistics and interpolation in GIS can accurately and precisely display multi-scale patterns in biovolume.
Since the early 1900s, the Everglades have been influenced by anthropogenic actions including altered hydrology and increased nutrient loading. In the northern Everglades an apparent effect of these disturbances has been the development and proliferation of dense cattail (Typha spp. ) stands in areas previously dominated by sawgrass (Cladium jamaicense Crantz) and sloughs. Cattail cover, soil nutrient concentrations, topography and fire history were determined for the Holey Land and Rotenberger Wildlife Management Areas, located in the northern Everglades. These data were analyzed using multiple regression to assess the relative influence of fire, hydrology and soil nutrients on cattail abundance. Holey Land and Rotenberger were overdrained over recent decades which resulted in soil compaction and nutrient accumulation, illustrated by increased soil bulk densities and elevated nutrient storage. Average bulk densities were 0.13 g cm−3 for Holey Land and 0.22 g cm−3 for Rotenberger. Average total P (TP) stored in the surface 10 cm of soil in Holey Land and Rotenberger were 7 and 13 g m−2, respectively. In contrast, Everglades soils uninfluenced by nutrient enrichment and with less severe overdrainage have bulk densities of 0.07 g cm−3 and TP storage of 4 g m−2. Typically, elevated soil P concentrations have been considered a primary factor influencing cattail growth and distribution in the Everglades. With the apparent absence of P limitation in Holey Land and Rotenberger, cattail abundance was influenced by either fire or hydrology. Forty-six percent of the variation of cattail cover in Holey Land was explained by elevation, indicating that increased water depth and duration of flooding have a significant impact on cattail expansion. In Rotenberger, fire was the most influential factor, explaining 57% of the variation in cattail cover. Hydrology was the second most important factor limiting cattail abundance.
The population dynamics of two temperate seagrasses, Zostera americana den Hartog and Z. marina L., were studied on an intertidal transect in Boundary Bay on the Pacific coast of Canada. Z. americana grew over most of the study area below the mean higher high water level, but Z. marina grew only below the mean lower low water level. Distributions most likely were restricted by tolerance of exposure to air and were modified by the irregular topography; e.g. two zones of dense Z. americana were separated by a channel in which Z. marina dominated.Rates of vegetative growth and flowering were not constant over the intertidal range of either seagrass. In May, vegetative shoots of Z. americana were more abundant and heavier in the lower intertidal zone than in the upper intertidal, but by the end of September the shoots were more abundant in the upper zone and the mean dry weight was constant over the transect. Flowering spread during the summer from a small portion of the upper zone (in May) both lower and higher on the transect; the heaviest reproductive shoots grew in the lower intertidal zone. In May, Z. marina had a high density and biomass of vegetative shoots in the mid-intertidal channel, but during the summer that population declined and plants lower on the transect flourished. Flowering, too, peaked earlier in the channel than in the lower intertidal zone. From early May to late September, Z. americana contributed 60–85% of the total shoots on the transect but only 37–69% of the dry weight. Overall, the density and biomass were less variable in the perennial Z. marina than in the annual Z. americana which yearly colonized many hectares from seed.
Cluster analysis, reciprocal averaging and a biotic boundary technique were used to analyze the relationship between community composition and topographic depth within 4 grass-sedge marshes in west-central Florida. Abundances of species fluctuated seasonally and varied between marshes. Communities in deep areas of individual marshes differed markedly from each other and from those in shallow areas. Reciprocal averaging showed no floristic gradients in the shallow marshes, but strong floristic gradients were highly correlated with water regime in deep marshes. Biotic boundaries within the marshes corresponded with water levels. The biotic boundary technique clearly delineated communities within the marshes and reflected the importance of seasonal water level fluctuations for community composition.
A total of 45 fish species and 55 decapod species were collected by seine net from the seagrass beds in Cockle Bay. The dominant fish species were Leiognathus bindus, Sillago maculata burrus, Favonigobius reichei, Lethrinus sp., and Pelates sexlineatus and these accounted for 87.5% of the total numbers. Dominant decapod taxa were Paratya, Rhynchocinetes and Brachycarpus. These were primarily small species or early juveniles of larger species. Temporal variation in both species composition and abundance was large: the peak number of fishes and decapods occurred in April 1999 and October 1999, whilst biomass was highest in April 1999. Fish and decapod numbers as well as biomass were lowest in August 2000. Temporal changes in the abundance of fishes and decapods corresponded with that of seagrass biomass and abundance of food organisms.
The marine macroalga, Ulva lactuca L. in its free-floating form was abundant throughout the year in the eutrophic inner parts of a Danish estuary and scarce in the outer parts. The growth rate of Ulva was not significantly different between the inner and outer parts. Grazing by invertebrates was negligible in the inner part and allowed biomass accumulation, whereas grazing pressure in the outer part matched the growth rate during summer and exceeded it by more than two-fold during autumn. Reduced grazing control is apparently an important and often overlooked factor for biomass accumulation of free-floating macroalgae under eutrophic conditions.
Sampling procedures represent a critical component of plant community studies, particularly for deep-water submersed species that are not easily observed. Here, two methods of submersed macrophyte sampling are compared for their accuracy and consistency. During 2 years of study, in-water sampling produced higher values of total species richness (nine species compared with eight in 1996 and 14 species compared with 10 in 1997), mean species richness in quadrats (2.3 compared with 1.4 in 1996, 2.2 compared with 1.1 in 1997) and frequency for all species than the boat surveys did. Small species were particularly vulnerable to underestimation in boat surveys. In-water sampling was also less variable, producing higher mean-to-variance ratios.
Distribution, abundance, and biomass data for seagrass communities at several locations on the coast of Oman are presented. The main study site was on the western side of Masirah Island on the Arabian Sea coast of Oman. This area is an important feeding ground for the green turtle, Chelonia mydas L., and it is affected by upwelling of low temperature waters during the summer monsoon. The depth distributions of Halodule uninervis (Forssk.) Aschers. and Halophila ovalis (R. Brown) Hook. f., the two most abundant seagrasses at this site, overlapped but were inversely related. Halodule dominated the intertidal zone and Halophila was more predominant in the deep subtidal, although total biomass of the two seagrasses were similar in this depth zone. At all depths, biomass of Halophila was about equally distributed between leaves and roots and rhizomes. Leaf biomass of Halodule was only 7–20% of the total biomass and the highest below-ground biomass occurred in the intertidal zone. Biomass of these species here and at other sites and of Thalassodendron ciliatum (Forssk.) den Hartog at this site was generally lower than comparative data in the Gulf and the Red Sea. Small patches of Syringodium isoetifolium (Aschers.) Dandy were also observed in Umm ar Rasas Bight making a total of four species recorded to occur in Oman. The reduced growth of seagrasses at Masirah Island seems to be due to stresses associated with the summer monsoon and grazing pressure. Survival of these populations is discussed in terms of seasonal growth and flowering.
Temporal patterns in plant cover and above and below-ground biomass were determined for tropical intertidal seagrasses at three coastal seagrass meadows in the Townsville region of northern Australia. Methods including monitoring of permanent intact quadrats using photography and a modified point quadrat technique, and harvesting adjacent quadrats, were employed to sample seagrasses at three-monthly intervals for two years. Abundance of total seagrass and individual seagrass species fluctuated seasonally by a factor of between two and four depending on the species, with minimum abundance in the dry season (August to September) and subsequent recovery of seagrass during the wet season months (November to March). There was some variation in this pattern among both sites and years. Correlations between seagrass abundance and climatic factors were examined via principal components analysis and multiple regression. Seagrass standing crop was positively correlated with daylength, temperature and rainfall events. However, the best model explained only 48 % of the variation. Temporal changes in seagrass community composition appeared consistent with successional theory.
A search for Halodule wrightii. Aschers. carried out along the most coast of Rio de Janeiro state revealed 21 populations. Populations were usually small, sparsely distributed and essentially monospecific. To quantify the intensity of seasonal variation within these southerly populations, samples were collected every 45 days at the site with most seagrass—Ilha do Japonês, Cabo Frio. Variation in biomass and density, and size distributions of short shoot lengths were measured. Short shoots made up between 21 and 44% of H. wrightii biomass. Seasonal changes in seagrass parameters (e.g. biomass from 12.1–37.3 g dry weight m-2, short shoot density from 2080–8482 m−2 and short shoot size from 28–106 mm), were strongly pronounced in the population between 1995 and 1997, although inter-annual variation in the measured parameters was also evident. The variation (coefficient of variation) of shoot size distributions also showed seasonal differences but differences in size skewness (skewness coefficient) were not found. No evidence of sexual reproduction was found. A comparison of the same variables in ten populations distributed along the coast of Rio de Janeiro state revealed substantial between site variation in shoot density, biomass distribution and short shoot size distributions. Biomass values for H. wrightii were relatively low. In the populations at Rio de Janeiro, rhizome biomass was positively correlated with root biomass, while shoot biomass was positively correlated with mean shoot length rather than shoot density.
The distribution of seagrasses in a 15-ha area in the mid-Indian River lagoon on Florida's central east coast was mapped. Halodule wrightii Aschers. dominated in shallow (< 0.4 m) and Syringodium filiforme Kutz. in deeper water (> 0.5 m). Thalassia testudinum Banks ex König occurred as scattered patches. Areal coverage of monospecific stands of the three major seagrasses was: Syringodium 35%, Halodule 14%, Thalassia 6% and bare sand 21%. Mixed species stands, mostly Syringodium with Hallodule, covered 25% of the total study area. Above-ground seagrass biomass was maximum in summer (June–July) and minimum in late winter (February–March). Summer maxima ranged from 60 g dry wt. m−2 for Syringodium to ∼ 300 g dry wt. m−2 for Thalassia, with Halodule intermediate at 160 g dry wt. m−2.Because distribution of unattached benthic macroalgae (“drift algae”), primarily Gracilaria spp., was highly aggregated, aggregations were first mapped, followed by stratified quadrat sampling in order to estimate total drift algal abundance. In April 1982, high-density patches covering a few hectares averaged 409 g dry wt. m−2. At maximum abundance, averaged over the entire 15-ha mapped area, drift algal biomass was 164 g dry wt. m−2; mean above-ground seagrass biomass was only 49 g dry wt. m−2. Other large expanses of the lagoon had similar accumulations of drift algae; densities of some accumulations exceeded 15 000 g dry wt. m−2. Year-to-year variability of seagrass and drift algal abundance was high and may be related to variations in light levels.Drift algae harbor high densities of animals and at times may be quantitatively more important locally than seagrasses in terms of habitat, nutrient dynamics and primary production.
From March 1998 to 1999, the spatial and temporal variation in shoot and abundance characteristics of the seagrass, Heterozostera tasmanica (Martins ex Aschers) den Hartog were examined in Westernport, an estuarine embayment in south-eastern Australia. Seagrass characteristics were investigated at three sites: Charing Cross; Crib Point; and Newhaven. In the north (Charing Cross), waters are highly turbid and subject to high nutrient inputs from catchment sources, while in the south (Newhaven) waters are primarily oceanic. Crib Point represents an intermediary region in water quality. Temporal patterns in shoot parameters (leaf weight per shoot, leaf and stem weight per shoot, number of clusters per shoot and leaf surface area per shoot) of H. tasmanica were highly site dependent and were generally 2–4-fold higher at Newhaven compared to Charing Cross; consistent with improved water quality from north to south in Westernport. Seagrass abundance parameters (above-ground biomass and shoot density) also differed between sites with 1–2-fold higher values at Newhaven relative to the other sites. Relationships between shoot and physico-chemical parameters indicate that light availability is the primary factor influencing the growth of intertidal H. tasmanica meadows in Westernport.