Qualitative and quantitative aspects of the epiphytic component in a mixed seagrass meadow from Papua New Guinea

During 1982, structural and functional aspects of the epiphytic component in a tropical mixed seagrass meadow, have been investigated for each seagrass species separately. This meadow consisted of the seagrasses Thalassia hemprichii (Ehrenb.) Aschers., Cymodocea serrulata (R.Br.) Aschers. et Magnus, C. rotundata Ehrenb. et Hempr. ex Aschers., Halodule uninervis (Forssk.) Aschers. and Syringodium isoetifolium (Aschers.) Dandy.No significant differences were observed in floristic composition, number of algal species, abundance and diversity of the epiphytic component. On an area basis, annual mean above-ground biomass (seagrass leaves and epiphytes), amounted to 82 g ADW, of which 18% could be ascribed to the epiphytic component. The contribution of the epiphytic component to the annual mean above-ground production ranged from 16% on leaves of Thalassia hemprichii to 33% on leaves of Cymodocea serrulata. Total annual mean epiphyte production was 4.6 g ADW m⁻² sediment surface day⁻¹ (19%).When including the macroalgal component of this mixed seagrass meadow, total annual mean above-ground plant biomass amounted to 93 g ADW (212 g DW) on an area basis, of which the epiphytes contributed 15.5% (28.5% DW), the macroalgal component 12% (32.5% DW) and the seagrass leaves 72.5% (39.5% DW). Aspects of the epiphytic component (e.g., floristic composition, abundance, biomass and production) in monospecific and mixed seagrass communities are discussed.     

Correlations between salinity and growth of the seagrass Amphibolis antarctica (labill.) Sonder & Aschers., In Shark Bay,Western Australia,using a new method for measuring production rate

A method of estimating above-ground productivity in situ of the seagrass Amphibolis antarctica (Labill.) Sonder & Aschers. has been devised, using tags to determine rates of leaf turnover. This has proved an effective tool in establishing the behavior of the species in relation to the gradient of increasing salinity which is present in Shark Bay. No seagrass was found beyond 64%_o, but measurement of production and biomass within dense patches of seagrass at different salinities revealed that these were at a maximum at a salinity of 42%_o, decreasing as the salinity increased and also at lower oceanic concentrations. Production rates ranged from 2 to 17 g dry weight m^?2 day^?1 with biomass from 600 to 2000 g m^?2, thus Amphibolis antarctica is one of the more productive Australian seagrass species, even in the hypersaline conditions of the Bay. Despite the obvious correlation between above-ground production and salinity, it is pointed out that the results are not taken to imply causality.     

Aspects of production and biomass of four seagrass species (Cymodoceoideae) from Papua New Guinea

Biomass and production data of the seagrasses Cymodocea serrulata (R. Brown) Aschers. and Magnus, Cymodocea rotundata Ehrenb. et Hempr. ex Aschers., Halodule uninervis (Forssk.) Aschers. and Syringodium iksoetifolium (aschers.) Dandy were collectede in monospecific stands in Bootless Inlet, Papua New Guinea. Cymodocea serrulata and Cymodocea rotundata were studied from November 1980 to November 1981. Total annual mean biomass was 354 and 201 g ADW m⁻², respectively. The largest proportion of these biomass values was contributed by the rhizomes (49 and 36%, respectively) and leaf biomass was ± 30% for both species. Halodule uninervis was studied at an intertidal and a subtidal site. The highest total annual mean biomass (600 g ADW m⁻²) was recorded at the intertidal site, of which 85% was found below ground. The largest proportion of the biomass, at both sites, was contributed by the below-ground vertical axes of the shoots. The biomass of the rhizomes was relatively low (9–12%) for Halodule uninervis. Proportionally, the largest above-ground biomass (40%) was recorded for Syringodium isoetifolium, of which the annual mean biomass was 481 g ADW m⁻².Total production (above and below ground) was 4.9 and 3.0 g ADW m⁻² day⁻¹ for Cymodocea serrulata and Cymodocea rotundata, respectively. Approximately 70% was production of leaves. Total production amounted to 6.0 and 4.0 g ADW m⁻² day⁻¹ for Halodule uninervis at the intertidal and subtidal sites, respectively. The maximum production was recorded for Syringodium isoetifolium, 60% of the 9.0 g ADW m⁻² day⁻¹ was contributed by the leaves. All species reached the maximum production during February and March, when the water temperatures were highest and water was retained above all sites, at all times. The increase of leaf production was mainly due to the increase in biomass of the mature leaves. Significant changes in the plastochrone interval of the leaves were not observed during this period.     

A comparison of the annual production and biomass in three monospecific stands of the seagrass Thalassia hemprichii (Ehrenb.) aschers.

Annual production and biomass data were collected in three seagrass communities of Thalassia hemprichii (Ehrenb.) Aschers. from Papua New Guinea. Leaf growth rates, determined by the marking technique, resulted in a growth rate of 8.3 mm day^?1 for the youngest leaves. Production of above-ground plant parts was assessed by the plastochrone interval. The annual mean values were 9.3, 10.0 and 9.9 days for Sites 1, 2 and 3, respectively. Annual mean total above-ground production amounted to 2.1 mg ADW shoot^?1 day^?1 at Site 1, and 5.5 and 4.5 mg ADW shoot^?1 day^?1 for Sites 2 and 3, respectively; 73–89% of the total net production was contributed by the leaves. Rhizome production was correlated to the plastochrone interval of the leaves. Annual mean biomass of leaves amounted to 16–27% of the total biomass. The mean biomass of the other plant parts remained constant during the year. The annual mean turnover time of the different plant parts (above- and below-ground) varied considerably between the sites.     

Quantitative and dynamic aspects of a mixed seagrass meadow in Papua New Guinea

Aspects of production and biomass were studied from November 1981 to November 1982 in six seagrass species which together from the mixed seagrass meadows in Papua New Guinea. These species, viz. Thalassia hemprichi (Ehrenb.) Aschers., Cymodocea serrulata (R.Br.) Aschers. et Magnus, Cymodocea rotundata Ehrenb. et Hempr. ex Aschers., Syringodium isoetifolium (Aschers.) Dandy, Halodule uninervis (Forssk.) Aschers. and Halophila ovalis (R.Br.) Hook. f. have been previously studied in monospecific seagrass beds. Thalassia hemprichii was the dominant species, followed by Syringodium isoetifolium. These two species were present in all samples and evenly distributed. Cymodocea serrulata and C. rotundata were recorded in 91 and 86%, respectively, of the quadrats sampled. The density, however, varied considerably. Shoots of the remaining two species were found in < 50% of the samples. The percentage presence increased when below-ground plant parts were taken into account.Significant differences in the shoot density were only found in Syringodium isoetifolium. The distribution of the five other species remained unchanged during the year. Annual mean shoot density amounted to 860 for Thalassia hemprichii, 2100 for Syringodium isoetifolium, 200 for Cymodocea serrulata, 250 for C. rotundata and 54 for both Halodule uninervis and Halophila ovalis. All species reached their maximum density from September to November. The mean aboveground production was 3.9 g ash-free dry weight (ADW) m⁻² day⁻¹, of which 64% was contributed by Thalassia hemprichii. Syringodium isoetifolium, which had the highest shoot density, contributed only 17%.The plastochrone interval of the leaves (PIL) was constant in all species and the mean ranged from 10.1 to 11.1 days. The PIL was virtually the same in this mixed meadows as in monospecific seagrass beds. Furthermore, the above-ground relative growth rate was constant during the year. Thalassia hemprichii was the most productive seagrass (mean 0.043 day⁻¹), whereas the lowest mean relative production was observed for Syringodium isoetifolium (0.030 day⁻¹). Total mean production was 6.4 g ADW m⁻² day⁻¹, of which 39% was contributed by the vertical axes, the rhizomes and the roots. The caloric production efficiency of the meadows was 0.58% of the total insolation at the water surface.Thalassia hemprichii was, because of its morphology, the stable element in the meadow. All other species were present at all times and exhibited a continuative process of recolonization.     

Annual biomass and production of epiphytes in three monospecific seagrass communities of Thalassia hemprichii (Ehrenb.) Aschers

The biomass of epiphytes and seagrasses has been measured in relation to leaf age in three monospecific seagrass stands of Thalassia hemprichii (Ehrenb.) Aschers. in Papua New Guinea. From June 1981 through August 1982, biomass values for epiphytes at the three sites ranged from 5 to 70 g ADW m⁻² sediment surface at site 1, from 5 to 14 g ADW m⁻² at site 2, and from 3.5 to 7.0 g ADW m⁻² at the site 3. Annual mean epiphyte biomass values for the different sites were 1.3 g ADW m⁻² leaf surface at site 1, 1.7 g ADW m⁻² leaf surface at site 2, and 1.5 g ADW m⁻² leaf surface at site 3.

The annual mean standing crop of T. hemprichii leaves was highest at site 1 (103 g ADW m⁻². Values for site 2 and site 3 were 60 g ADW m⁻² and 41 g ADW m⁻², respectively.

Production of epiphytes was calculated in three different ways: firstly, by using biomass values for each specific leaf-age group, with corrections for colonization; secondly, by fitting the biomass values with a specific growth curve; and thirdly, by estimated the rate of biomass accumulation. On an area basis, production of epiphytes on leaves of T. hemprichii ranged from 0.55 to 3.97 g ADW m⁻² day⁻¹ at site 1, from 0.17 to 0.73 g ADW m⁻² day⁻¹ at site 2, and from 0.24 to 0.68 g ADW m⁻² day⁻¹ at site 3.     

Seasonal abundance and distribution of drift algae and seagrasses in the mid-Indian river lagoon,Florida

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.     

A preliminary study of the seagrass Thalassodendron ciliatum (Forssk.) den hartog from Eastern Indonesia. Biological results of the snellius II expedition

Two dense meadows of the seagrass Thalassodendron ciliatum (Forssk.) den Hartog were sampled during the Indonesian—Dutch Snellius II expedition to Eastern Indonesia. Production data were obtained from one of these meadows. The production of leaf biomass was measured by the leaf marking technique of Zieman and by the plastochrone interval method. The two methods reached comparable results. The production of leaf tissue was 4.2 mg ADW shoot^?1 day^?1. The production of rhizome biomass was calculated in a similar way, based on the plastochrone interval of rhizome nodes. The production of the meadow, exclusive of the production of roots and fruits, amounted to 4.5 g ADW m^?2 day^?1. A significant correlation between the growth rates of rhizomes and leaves was observed. Biomass data from the second site are given.     

Biomass accumulation and shading effects of epiphytes on leaves of the seagrass, Heterozostera tasmanica, in Victoria,Australia

A method is described for estimating the rate of accumulation of epiphyte biomass on leaves of the seagrass, Heterozostera tasmanica (Martens ex Aschers.) den Hartog and for estimating the effect of epiphyte biomass on photosynthesis of the seagrass. Epiphyte biomass was determined by comparison of the weight per unit area of epiphyte-covered and epiphyte-free leaf blades. Epiphyte weight increased as age of the seagrass leaves increased. Linear regression on epiphyte biomass vs. leaf age estimated the rate of biomass accumulation. Rates varied from 5.7 to 104 μg epiphyte dry weight per cm² of leaf surface per day at three sites in Western Port and Port Phillip Bay, Victoria. Rates of accumulation of epiphyte biomass were generally higher during December through March (summer) than in May (autumn), August (winter) or October (Spring). Light attenuation by epiphytes increase linearly with biomass. The rate of biomass accumulation of epiphytes was compared with leaf growth rate, ambient photon flux density in H. tasmanica beds and the photosynthesis—photon flux density curve of H. tasmanica. This comparison demonstrated that epiphyte biomass can accumulate fast enough to shade H. tasmanica leaves and significantly reduce the time (to less than one half of the leaf life span) in which positive net photosynthesis of the leaf blade is possible.     

Effect of nutrient availability, grazer assemblage and seagrass source population on the interaction between Thalassia testudinum (turtle grass) and its algal epiphytes

Seagrass leaves are often densely covered by epiphytic algae which can suppress seagrass productivity and has been implicated in declines of seagrass meadows worldwide. The net effect of epiphytes on seagrass growth and morphology depends on the independent and interactive effects of a variety of factors, including nutrient availability and the intensity of grazing on epiphytes. Here I report the results of a mesocosm experiment designed to test the effects of nutrient addition and within-functional group variation (grazer species composition and the source population of seagrass) on the strength of the interactions among grazers, epiphytes, and turtle grass (Thalassia testudinum). Turtle grass ramets from two sites in the northern Gulf of Mexico were cleared of epiphytes and transplanted into common-garden mesocosms. Replicate ramets were grown in a split-split plot design with two levels of dissolved nutrients and four different grazer species combinations (Tozeuma carolinense alone, Pagurus maclaughlinae alone, both species together, and no grazers present). As expected, grazers had a significant negative effect on epiphyte biomass/leaf area and a significant positive effect on turtle grass growth in the mesocosms. The two species were more similar in their direct effects on epiphyte biomass than in their indirect effects on turtle grass growth; this may reflect differences in epiphyte community composition under different grazer treatments. The effect of nutrient addition on turtle grass growth depended critically on the intensity of grazing: in the presence of grazers, turtle grass tended to produce a greater biomass of new leaf tissue in the tanks with nutrients added than in the control tanks. However, when grazers were absent, the direction of the effect was reversed, and plants with nutrients added grew less than the control plants. The two source populations of turtle grass differed significantly in epiphyte biomass/leaf area accrued in the mesocosms as well as in the strength of the effect of grazers on turtle grass growth. This suggests that population differentiation in seagrass interactions with epiphytes, as well as spatial and temporal variation in resources and grazer community composition, can greatly effect the role of epiphytes in limiting seagrass productivity.     

The seasonal distribution and community structure of the epiphytic algae on Thalassia hemprichii (Ehrenb.) Aschers. from Papua New Guinea

Algae growing as epiphytes on leaves of Thalassia hemprichii (Ehrenb.) Aschers. have been studied from November 1980 to December 1981, in the Port Moresby area, Papua New Guinea. The epiphytic communities of 3 different monospecific seagrass meadows are compared for species richness, abundance and temporal pattern. Seagrass shoots were studied separately, using the method of Braun—Blanquet, as adapted by Boudouresque. By differentiating between the leaves of one single shoot, the inner- and outer-face of each leaf and the upper- and lower-part of each leaf, the epiphytic community was studied from its initial colonization (Leaf 1) to the final “climax” situation (Leaf 4). The diversity and abundance were strongly related to the age of the seagrass leaves. The Rhodophyta were best represented, with the Cryptonemiales dominating the community quantitatively; the Ceramiales predominated qualitatively. The Phaeophyta were negligible in terms of abundance and diversity. Differences between the 3 study sites are presented.     

Seagrass response to CO2 contingent on epiphytic algae: indirect effects can overwhelm direct effects

Increased availability of dissolved CO₂ in the ocean can enhance the productivity and growth of marine plants such as seagrasses and algae, but realised benefits may be contingent on additional conditions (e.g. light) that modify biotic interactions between these plant groups. The combined effects of future CO₂ and differing light on the growth of seagrass and their algal epiphytes were tested by maintaining juvenile seagrasses Amphibolis antarctica under three different CO₂ concentrations representing ambient, moderate future and high future forecasts (i.e. 390, 650 vs. 900 µl l^?1) and two light levels representing low and high PAR (i.e. 43 vs. 167 µmol m^?2 s^?1). Aboveground and belowground biomass, leaf growth, epiphyte cover, tissue chemistry and photosynthetic parameters of seagrasses were measured. At low light, there was a neutral to positive effect of elevated CO₂ on seagrass biomass and growth; at high light, this effect of CO₂ switched toward negative, as growth and biomass decreased at the highest CO₂ level. These opposing responses to CO₂ appeared to be closely linked to the overgrowth of seagrass by filamentous algal epiphytes when high light and CO₂ were combined. Importantly, all seagrass plants maintained positive leaf growth throughout the experiment, indicating that growth was inhibited by some experimental conditions but not arrested entirely. Therefore, while greater light or elevated CO₂ provided direct physiological benefits for seagrasses, such benefits were likely negated by overgrowth of epiphytic algae when greater light and CO₂ were combined. This result demonstrates how indirect ecological effects from epiphytes can modify independent physiological predictions for seagrass associated with global change.     

Epiphytic algae on eelgrass at Roscoff,France

Algae growing as epiphytes on eelgrass, Zostera marina L., at Roscoff were investigated during one year by examining the species richness and abundance. The abundance of the algae, expressed as cover values, appeared to be influenced by the characteristics of the substrate, especially the ephemeral status and growth rate of the leaves.Colonization of the seagrass leaves occurred primarily on tips and edges, subsequently spreading over the faces to the base. The pioneers appeared to be the dominant, persistent taxa, of which Audouinella virgatula (Harv.) Dixon, Fosliella lejolisii (Rosan.) Howe, Myrionema magnusii (Sauv.) Lois. and the diatoms were most prominent. The oldest parts of fully grown leaves showed the most diverse epiphytic vegetation. Though there was a form of succession, a climax community was not reached.On examining the spatial distribution of the epiphytes, a group of characteristic taxa was seen to be accompanied by a second group, whose variation in occurrence seemed to be correlated with the height in the littoral.     

Aspects of seagrass ecology along the western Arabian Gulf coast

Results of semi-quantitative observations and quantitative sampling of seagrasses at coastal and offshore sites along the western Arabian Gulf are presented. Overall seagrass cover (all species together) shows significant positive correlation with latitude, but not with salinity, temperature or depth. The same pattern is shown by Halodule uninervis (Forsk.) Aschers., the dominant species. Mean seagrass biomass ranged from 53–235 g m^-2 (dry weight). These values are comparable with biomass estimates from regions in which environmental conditions are generally less extreme than in the Gulf. Seagrass biomass is significantly negatively correlated with depth and sediment grain size. No significant correlation is apparent between seagrass biomass and factors such as season, salinity, or concentrations of nutrients and heavy metals measured. It is pointed out that any correlations observed are not necessarily taken to imply causality.     

Macrophyte-epiphyte biomass and productivity in an eelgrass (Zostera marina L.) community

The biomass, productivity (¹⁴C), and photosynthetic response to light and temperature of eelgrass, Zostera marina L. and its epiphytes was measured in a shallow estuarine system near Beaufort, North Carolina, during 1974. The maximum of the biomass (above-ground) was measured in March; this was followed by a general decline throughout the rest of the year. The average biomass was 105.0 g dry wt m^?2; 80.3 g dry wt m^?2 was eelgrass and 24.7 g dry wt m^?2 was epiphytes. The productivity of eelgrass averaged 0.88 mg C g^?1 h^?1 which was similar to that of the epiphytes, 0.65 mg C g^?1 h^?1. Eelgrass and epiphyte productivity was low during the spring and early summer, gave a maximum during late summer and fall, and declined during the winter; this progression was probably due to environmental factors associated with tidal heights. On an areal basis, the average annual productivity was 0.9 g C m^?2 day^?1 for eelgrass and 0.2 g C m^?2 day^?1 for the epiphytes. Rates of photosynthesis of both eelgrass and epiphytes increased with increasing temperature to an asymptotic value at which the system was light saturated. Both eelgrass and epiphytes had a temperature optimum of < 29 °C. A negative response to higher temperatures was also reflected in biomass measurements which showed the destruction of eelgrass with increasing summer temperatures. The data suggest that the primary productivity cycles of macrophytes and epiphytes are closely interrelated.     

Are epiphytes a significant component of intertidal Zostera noltii beds?

The role of epiphytes in an intertidal Zostera noltii seagrass bed in Marennes-Oléron Bay was assessed in comparison with the other main benthic primary producers (Z. noltii, microphytobenthos) at two bathymetric levels and on a seasonal basis. Assemblage and biomass of epiphytes were studied using scanning electron microscopy (SEM). Z. noltii and its detrital matter followed a typical seasonal pattern: microphytobenthos was present in large quantities throughout the year representing 21% of the total biomass while detrital matter, above-ground parts and below-ground parts accounted for 65, 9 and 5%, respectively. Only two species of epiphytic diatoms, Cocconeis scutellum and Cocconeis placentula, were observed on seagrass leaves. Epiphyte biomass was very low, representing on average less than 0.001% of that of microphytobenthos or leaves. This low epiphyte biomass is linked with the absence of macroalgae and also with the low biovolume of Cocconeis, which formed a monolayer of cells on leaves. This can be explained by the severe conditions of the intertidal and the high leaf turn-over of Z. noltii leaves.     

Seasonal variation in standing crop,density and leaf growth rate of the seagrass, Heterozostera tasmanica, in western Port and Port Phillip Bay,Victoria, Australia

Standing crop, density and leaf growth rate of Heterozostera tasmanica (Martens ex Aschers.) den Hartog along with light, temperature, nutrient and sediment characteristics were determined monthly for fifteen months at three study sites in Western Port and one site in Port Phillip Bay, Victoria, Australia. Erect vegetative stems of H. tasmanica were frequently branched, were present throughout the year and accounted for 25–60% of the above-sediment biomass, with the stem proportion higher during winter than summer. At three of the four sites there was a unimodal seasonal pattern in which minimum leaf standing crop (27–61 g dry wt. m^?2), density (600–2000 leaf cluster m^?2) and leaf productivity (0.34–0.77 g dry wt. m^?2 day^?1) generally occurred during winter (June–August) and maximum leaf standing crop (105–173 g dry wt. m^?2), density (2700–5000 leaf cluster m^?2) and leaf productivity (2.6–4.2 g dry wt. m^?2 day^?1) occurred during summer (December–February). A bimodal seasonal pattern with minimum standing crop and density during midsummer occurred at one site. This anomalous seasonal pattern may be due to exposure and desiccation stress during spring low tides. At the site receiving the lowest irradiance, standing crop, density and annual leaf production also were lowest, but length and width of leaves, shoot height and leaf growth rate per leaf cluster were the highest of the four study sites. On average, each leaf cluster at any one of the study sites produced 30–31 leaves per year with mean leaf turnover rates of 1.3–1.7% day^?1. Annual leaf production of H. tasmanica ranged from 410 to 640 g dry wt.m^?2 at the four sites.     

Detection of inconspicuous epiphytic algae supporting food webs in seagrass meadows

Summary Detritus from common seagrasses and other marine angiosperms may often be a less important basis for estuarine food webs than previously believed. In NW Gulf of Mexico seagrass meadows, epiphytic algae have high productivities, palatability, and a more important trophic role than common large plants have. Interdisciplinary field experiments show (1) intensive night-time ingestion of epiphytes by various invertebrate detritivores, (2) very high productivity of epiphytic algae on seagrasses, and (3) assimilation of epiphytes rather than seagrasses, as measured by ¹³C comparisons. These combined data show that many naturally concentrated and potentially competing invertebrates in Gulf of Mexico seagrass meadows feed largely on the algal overgrowth on seagrass blades, even when such algae appear to be sparse. Primary productivity of these epiphytic algae can equal that of the seagrasses, per blade or per unit biomass. Animal ¹³C values tracked epiphytic values rather than seagrass values when comparisons were made over six sites. These measurements reinforce the view that epiphytic algae can be the primary basis of the food web in seagrass meadows.Contribution No. 608 of The University of Texas Marine Science Institute     

Seasonal shifts in seagrass bed primary producers in a cold-temperate estuary: Dynamics of eelgrass Zostera marina and associated epiphytic algae

The seasonal dynamics of seagrass and epiphytic algal primary production were measured in an eelgrass (Zostera marina) bed in the Akkeshi-ko estuary, Hokkaido, Japan (43°02′N, 144°52′E). During spring and early summer, eelgrass biomass increased, with a high production (maximum: 2.89 g C m⁻² day⁻¹), but the production and biomass of epiphytic algae remained low. In contrast, epiphytic algae bloomed in August, with a high production (5.21 g C m⁻² day⁻¹), but eelgrass production ceased and its biomass subsequently decreased. Therefore, the major primary producers in this eelgrass bed switched seasonally from eelgrass in spring and early summer to epiphytic algae in late summer and autumn. Epiphytic algae maintained similar productivity because of the change of photosynthetic kinetics and the dominant epiphytic diatom changed from highly adhesive species to less adhesive or filamentous small species during the bloom. This suggests that the change of epiphyte density and biomass was due to change of its loss rate, possibly due to herbivorous grazing rate. Moreover, competition between epiphytic algae and eelgrass for nutrients and light may also affect the dramatic seasonal changes in the major primary producers.