Regulation of spatial and seasonal variation of macroalgal biomass in a brackish, eutrophic lake

Processes leading to biomass variation of Ulva were investigated at two contrasting sites in the eutrophic Veerse Meer (The Netherlands). Ulva species dominated at the Middelplaten site, while at the Kwistenburg site a mixture of Ulva spp. and Chaetomorpha linum dominated. Total summer macroalgal biomass was higher at Middelplaten than at Kwistenburg (282 and 79 g DW m^–2, respectively). Growth rates of Ulva spp. were high at both sites in May 1992 (cage mean 0.28–0.30 day^–1), but quickly dropped to lower values (0.05–0.10 day^–1). In May, growth rates were significantly highest at Kwistenburg, while during the rest of the season growth rates were similar for both sites. Temperature, pH, dissolved oxygen, salinity, light attenuation, phytoplankton and nutrient concentrations did not differ between sites. The relation between variation in Ulva spp. growth rates and environmental parameters was analysed using stepwise multiple regression, showing that light and temperature were the main variables regulating Ulva spp. growth rates. As Ulva growth rates were similar for both sites but biomass was much lower at Kwistenburg it was concluded that a large amount of produced biomass was lost at Kwistenburg. Although the exact reason for this remains unclear, it seems most likely that transport of macroalgae by wind and waves is a very important factor. This study shows the importance of simultaneously measuring growth rates and biomass at a high temporal resolution to reveal the mechanisms responsible for spatial variation in macroalgal biomass in shallow coastal areas. Electronic Publication     

Sedimentation in Arctic Canada: Species composition and biomass of phytoplankton contributed to the marine sediments in Frobisher Bay

Summary Sedimentation of phytoplankton provides food and energy for zoobenthic communities. In this study the rates, species composition and biomass of phytoplankton input to Frobisher Bay sediments were examined during ice (late November to July) and open water (late July to October) periods from 1982 to 1985. The rates were higher on the sea bed than at 20 m. The minimum rate (3x10⁵ cells·m^-2·day^-1) of sedimentation occurred during the early part of the ice period. It increased as the ice thickened and reached a maximum of 2.8x10⁸ cells·m^-2·day^-1 after the phytoplankton bloom at the beginning of the open water period in the first two weeks of August. The sedimented phytoplankton was dominated by diatoms, with a great majority of pennate species during the spring (April to June) and centric forms during the summer (July to August). Green flagellates, dinoflagellates and chrysophytes occurred as a low percentage of the total population in all seasons. Other indicators (chlorophyll a and phaeopigments) showed highest biomass levels in the deepest traps. They were consistently low during the winter (December to March) and reached their maxima during the open-water period of summer. Their abundance was correlated with the seasonal cycle of the phytoplankton in the water column.     

Build-up and decline of summer phytoplankton biomass in the eastern Weddell Sea,Antarctica

The seasonal development and decline of phytoplankton was investigated in the eastern Weddell Sea during summer and fall 1991. During the first half of the study (15 Jan–13 Feb) in an area off Vestkapp, favourable irradiance/mixing regimes initiated net phytoplankton growth in ice-free waters on the shelf and in stretches of open water over the partially ice-covered deep ocean. Chi a concentrations in the upper water column were moderate (0.2–0.8 g l^–1), but significantly above winter values. Later in the season (16 Feb–11 March), a phytoplankton bloom with surface Chl a concentrations ranging from 1.6–2.3 g l^–1 was encountered in an area further to the east. We suggest that the upper water column must have been stratified in this region for time scales of weeks to faciliate bloom development. Bacterial biomass and productivity generally paralleled the seasonal development of the phytoplankton. Nitrate concentrations in the upper mixed layer were substantially lower than would be expected from the existing phytoplankton standing stock, suggesting that heterotrophic consumption of organic matter by bacteria and zooplankton removed a large fraction of the primary production. The shallow seasonal pycnocline was eventually eroded by the passage of a storm, resulting in a homogeneous distribution of phytoplankton biomass over the entire water column, followed by sedimentation and deposition of phytodetritus on the sea floor. After the storm induced destratification, bacterial productivity was particularly high, amounting to more than half of the primary production (range: 10%–120%) in the upper water column. Subsequently, phytoplankton biomass in the upper water column decreased to values <1 g Chl a l^–1. The combination of low incident irradiances and incessant deep mixing prevented the phytoplankton biomass to increase again. During the last week of the investigation, extensive new-ice formation was observed. A major fraction of the residual surface plankton was incorporated into new sea ice, thus terminating the pelagic growth season of the phytoplankton in the eastern Weddell Sea.     

In situ growth rate and distribution of the ichthyotoxic dinoflagellate Gyrodinium corsicum Paulmier in an estuarine embayment (Alfacs Bay, NW Mediterranean Sea)

Gyrodinium corsicum Paulmier produced massive blooms in Alfacs Bay (Ebro Delta, NW Mediterranean Sea) from December 1994 to April 1995, a period characterized by an initial period of water stability and low outward flux of water during subsequent months. The highest cell densities of G.corsicum were found at the bottom of the bay during the population maintenance period (from January to March). During the day, no differences in the vertical distribution of G.corsicum were observed and no diel in situ vertical migration of the population was recorded. The in situ growth rate of G.corsicum was estimated using the cell cycle method since the cell division cycle was well phased with the daily light cycle. The S phase fraction of G.corsicum reached a maximum near the middle of the dark period. The G2M peak usually occurred in the dark period. The duration of the S phase was 2.1-8.7 h and the duration of the G2M phase was 2.6-7.7 h. The estimated specific in situ growth rate of G.corsicum ranged from 0.94 day^-1 during the initial phases of the bloom to 0.3 day^-1 when the bloom was well developed. Growth rates, measured in different locations at the same time, varied by a factor of 1.5, suggesting that different parts of the same blooming population were at different stages of growth. The persistence of the G.corsicum bloom with high cell densities (10⁵-10⁶ cells l^-1) at the bottom of the bay is discussed in relation to the low cell losses by physical dispersion in this area and the lack of upward vertical migration of G.corsicum.      

Response of Sargasso Sea phytoplankton biomass, growth rates and primary production to seasonally varying physical forcing

The response of phytoplankton biomass, growth rates and primaryproduction to seasonally varying physical forcing was studiedat a station southeast of Bermuda over an 18 month period. Phytoplanktongrowth rates and primary production were measured using thepigment-labeling method, and phytoplankton biomass was calculatedfrom these measurements. Phytoplankton carbon biomass variedsystematically over the year. Highest values were observed duringthe winter and spring. Seasonal variations of chlorophyll (Chi)a in the surface layer could primarily be attributed to variationsin phytoplankton biomass and secondarily to photoacclimation.During the summer period, average values of carbon (C)/Chl ratios(g C g^–1 Chi) ranged from 160 at the surface to 33 atthe 1.6% light level, changes attributed to photoacclimationof the phytoplankton, consistent with the observation that phytoplanktonbiomass did not vary as a function of depth. Phytoplankton growthrates in the surface layer did not vary systematically overthe year, ranging from 0.15 to 0.45 day^–1, in spite ofseasonally varying concentrations of nitrate. Growth rates variedas a function of depth from average values of 0.3 day^–1in the surface layer to <0.1 day¹ at the 1.6% light level.Thus, the primary response of the phytoplankton community tonutrient enrichment during the winter period was an increasein phytoplankton biomass rather than an increase in growth rates.A simple nutrient-phyto-plankton-zooplankton model was usedto explore this phenomenon. The model demonstrated that theobserved response of the phytoplankton to nutrient enrichmentis only possible when phytoplankton growth is not severely limitedby nutrients.     

Natural growth and diet of known‐age pallid sturgeon (Scaphirhynchus albus) early life stages in the upper Missouri River basin,Montana and North Dakota

Prior to anthropogenic modifications, the historic Missouri River provided ecological conditions suitable for reproduction, growth, and survival of pallid sturgeon Scaphirhynchus albus. However, little information is available to discern whether altered conditions in the contemporary Missouri River are suitable for feeding, growth and survival of endangered pallid sturgeon during the early life stages. In 2004 and 2007, nearly 600 000 pallid sturgeon free embryos and larvae were released in the upper Missouri River and survivors from these releases were collected during 2004–2010 to quantify natural growth rates and diet composition. Based on genetic analysis and known‐age at release (1–17 days post‐hatch, dph), age at capture (dph, years) could be determined for each survivor. Totals of 23 and 28 survivors from the 2004 and 2007 releases, respectively, were sampled. Growth of pallid sturgeon was rapid (1.91 mm day^?1) during the initial 13–48 dph, then slowed as fish approached maximum length (120–140 mm) towards the end of the first growing season. The diet of young‐of‐year pallid sturgeon was comprised of Diptera larvae, Diptera pupae, and Ephemeroptera nymphs. Growth of pallid sturgeon from ages 1–6 years was about 48.0 mm year^?1. This study provides the first assessment of natural growth and diet of young pallid sturgeon in the wild. Results depict pallid sturgeon growth trajectories that may be expected for naturally produced wild stocks under contemporary habitat conditions in the Missouri River and Yellowstone River.     

Seasonal dynamics of picophytoplankton in Lake Kinneret, Israel

1. Picophytoplankton (picocyanobacteria and picoeukaryotes) communities in Lake Kinneret were studied from 1988 to 1992. No prochlorophytes were observed in the lake. 2. Picocyanobacteria were a prominent and ubiquitous component of the phytoplankton, being present at all depths throughout the year, with concentrations ranging from 2 ± 10–8 ± 10⁵ cells ml^?1. Low cell numbers in winter and spring were followed at the end of the annual dinoflagellate bloom by maximal abundances in summer-autumn in the epilimnion. High cell numbers (> 10⁴ cells ml^?1) were sometimes also found in the anaerobic hypolimnion. Net growth rates for picocyanobacteria ranged from 0.29 to 0.60 divisions day^?1. 3. Picoeukaryotes were a very minor constituent of the picoplankton, mostly present in winter and spring, and sometimes at the end of autumn, with concentrations ranging from 44 to 5700 cells ml^?1. Higher cell numbers tended to occur in the near surface water layers. In August-September, picoeukaryotes were found only in the hypolimnion. In December, the occurrence of picoeukaryotes in the deep water layers probably resulted from advection with cold water currents from the Jordan river. Net growth rates for picoeukaryotes ranged from 0.26 to 0.43 divisions day^?1. 4. Overall, the contribution of picophytoplankton to the phytoplankton standing crop in Lake Kinneret was limited; picocyanobacteria and picoeukaryotes accounted for no more than 7.0 and 0.1% of total algal biomass (semiannual average), respectively. 5. Picophytoplankton cell numbers in pelagic waters were usually similar to those in shallower lake stations. 6. Picocyanobacteria appear to be an autochthonous population, whereas picoeukaryotes are probably brought annually by the Jordan River and do not maintain themselves in the lake.     

Preliminary investigation of the contribution of fast-ice algae to the spring phytoplankton bloom in Ellis Fjord,eastern Antarctica

 Algae released from fast-ice in Ellis Fjord, eastern Antarctica, made little contribution to subsequent phytoplankton growth. Dominant taxa in the interior ice community included Nitzschia cylindrus (Grun) Hasle, Navicula glaciei V.H. and a dinoflagellate cyst. Diatom mortality within the ice was high. The algal contribution to the phytoplankton from the fast ice was estimated by calculating the difference between algal biomass in ice cores taken on 14 November with those taken on 18 December 1992. The biomass of sedimenting phytoplankton was estimated using sediment traps; weekly cell counts of water were used to monitor net phytoplankton growth. The low contribution from the fast-ice of Ellis Fjord to the phytoplankton is similar to results from other Antarctic fast-ice communities but is not necessarily reflective of processes occurring within either Antarctic or Arctic pack ice communities. An algal mat growing on the base of the fast-ice had a carbon standing crop of between 0.231 gC m^-2 and 0.022 gC m^-2. Much of this was delivered to the water column as the ice melted while the remainder was exported. Received: 15 March 1995/Accepted: 4 September 1995     

Microbial processes in a high-latitude fjord (Kongsfjorden, Svalbard): II. Ciliates and dinoflagellates

The composition and ecological role of ciliates and dinoflagellates were investigated at one station in Kongsfjorden, Svalbard, during six consecutive field campaigns between March and December 2006. Total ciliate and dinoflagellate abundance mirrored the seasonal progression of phytoplankton, peaking with 5.8 × 10⁴ cells l⁻¹ in April at an average chlorophyll a concentration of 10 μg l⁻¹. Dinoflagellates were more abundant than ciliates, dominated by small athecates. Among ciliates, aloricate oligotrichs dominated the assemblage. A large fraction (>60%) of ciliates and dinoflagellates contained chloroplasts in spring and summer. The biomass of the purely heterotrophic fraction of the ciliate and dinoflagellate community (protozooplankton) was with 14 μg C l⁻¹ highest in conjunction with the phytoplankton spring bloom in April. Growth experiments revealed similar specific growth rates for heterotrophic ciliates and dinoflagellates (<0–0.8 d⁻¹). Food availability may have controlled the protozooplankton assemblage in winter, while copepods may have exerted a strong control during the post-bloom period. Calculations of the potential grazing rates of the protozooplankton indicated its ability to control or heavily impact the phytoplankton stocks at most times. The results show that ciliates and dinoflagellates were an important component of the pelagic food web in Kongsfjorden and need to be taken into account when discussing the fate of phytoplankton and biogeochemical cycling in Arctic marine ecosystems.     

Phytoplankton nitrogen demand and the significance of internal and external nitrogen sources in a large shallow lake (Lake Balaton,Hungary)

Since the middle of 1990s the trend of Lake Balaton towards an increasingly trophic status has been reversed, but N₂-fixing cyanobacteria are occasionally dominant, endangering water quality in summer. The sources of nitrogen and its uptake by growing phytoplankton were therefore studied. Experiments were carried out on samples collected from the middle of the Eastern (Siófok) and Western (Keszthely) basins between February and October 2001. Ammonium, urea and nitrate uptake and ammonium regeneration were measured in the upper 5-cm layer of sediment using the ¹⁵N-technique. Ammonium was determined by an improved microdiffusion assay. N₂ fixation rates were measured by the acetylene-reduction method. Ammonium regeneration rates in the sediment were similar in the two basins. They were relatively low in winter (0.13 and 0.16 μg N cm^?3 day^?1 in the Eastern and Western basin, respectively), increased slowly in the spring (0.38 and 0.45 μg N cm^?3 day^?1) and peaked in late summer (0.82 and 1.29 μg N cm^?3 day^?1, respectively). Ammonium uptake was predominant in spring in the Eastern basin and in summer in the Western basin, coincident with the cyanobacterial bloom. The amount of N₂ fixed was less than one third of the internal load during summer when external N loading was insignificant. Potentially, the phytoplankton N demand could be supported entirely by the internal N load via ammonium regeneration in the water column and sediment. However, the quantity of N from ammonium regeneration in the upper layer of sediment combined with that from the water column would limit the standing phytoplankton crop in spring in both basins and in late summer in the Western basin, especially when the algal biomass increases suddenly.     

Population dynamics of bacteria in Arctic sea ice

The dynamics of bacterial populations in annual sea ice were measured throughout the vernal bloom of ice algae near Resolute in the Canadian Arctic. The maximum concentration of bacteria was 6.0·10¹¹ cells·m^–2 (about 2.0·10¹⁰ cells·l^–1) and average cell volume was 0.473 m³ in the lower 4 cm of the ice sheet. On average, 37% of the bacteria were epiphytic and were most commonly attached (70%) to the dominant alga,Nitzschia frigida (58% of total algal numbers). Bacterial population dynamics appeared exponential, and specific growth rates were higher in the early season (0.058 day^–1), when algal biomass was increasing, than in the later season (0.0247 day^–1), when algal biomass was declining. The proportion of epiphytes and the average number of epiphytes per alga increased significantly (P<0.05) through the course of the algal bloom. The net production of bacteria was 67.1 mgC·m^–2 throughout the algal bloom period, of which 45.5 mgC·m^–2 occurred during the phase of declining algal biomass. Net algal production was 1942 mgC·m^–2. Sea ice bacteria (both arctic and antarctic) are more abundant than expected on the basis of relationships between bacterioplankton and chlorophyll concentrations in temperate waters, but ice bacteria biomass and net production are nonetheless small compared with the ice algal blooms that presumably support them.     

Are copepods important grazers of the spring phytoplankton bloom in the western Irish Sea?

The spring phytoplankton bloom and copepod grazing were studied at a coastal and offshore station in the western Irish Sea during 1997. Maximum chlorophyll standing stocks of 132.8 mg m^-2 inshore and 199.4 mg m^-2 offshore were measured in late April. At that time, mean water column temperatures were 10 and 8C at the coastal and offshore station, respectively. Spring bloom production at the coastal station was estimated as 31.2 g C m^-2 and was dominated by the diatom Guinardia delicatula. Offshore, production was 28.2 g C m^-2 and the bloom was composed of small (10 m) phytoflagellates and the silicoflagellate Dictyocha speculum. Maximum copepod abundance (189 and 544 x 10³ individuals m^-2, inshore and offshore, respectively) coincided with the spring bloom. Pseudocalanus and Temora ingestion rates were derived from measurements of gut pigment fluorescence, and were found to vary during the course of the spring bloom as a result of changes in gut content. Grazing by late copepodite and adult Pseudocalanus and Temora was variable inshore, but overall accounted for 17% of bloom production. Offshore, 22% of bloom production was grazed with maximum grazing (76% of daily production) occurring at the end of the bloom. Large copepod species were not major grazers of the spring bloom. Greater utilization of spring bloom production by copepods in the western Irish Sea compared to regions of the North Sea is attributed to differences in population size at the time of the bloom.      

Primary production and phytoplankton in two small,polyhumic forest lakes in southern Finland

Primary production and phytoplankton in polyhumic lakes showed a very distinct seasonal succession. A vigorous spring maximum produced by Chlamydomonas green algae at the beginning of the growing season and two summer maxima composed mainly of Mallomonas caudata Iwanoff were typical. The annual primary production was ca. 6 g org. C · m^–2 in both lakes. The mean epilimnetic biomass was 1.1 in the first lake and 2.2 g · m^–2 (ww) in the second one. The maximum phytoplankton biomass, 14 g · m^–2, was observed during the vernal peak in May.     

BIOMASS AND DYNAMICS OF GROWTH OF ULVA SPECIES IN PALMONES RIVER ESTUARY1

During the last decade, the Palmones River estuary has undergone severe eutrophication followed by a green tide episode; two species of Ulva, rotundata Blid. and Ulva curvata (Kütz.) De Toni, were the main macroalgae responsible for this bloom. From November 1993 to December 1994, we followed the biomass, the growth dynamics, and tissue elemental composition (C:N:P)of Ulva species, as well as some physicochemical variables in the estuary. Maximum biomass (up to 375 g dry wt·m^?2 in some spots, corresponding to a thallus area index of nearly 17 m²Ulva·m^?2 sediment) were observed in June and December. However, the biomass varied among the sampling stations. Water nitrate, ammonia, and phosphate showed high concentrations throughout the year, with extremely high transient pulses, sustaining the high growth rates observed. Growth rates were estimated directly in the field. The rates were generally higher in Ulva discs maintained in net cages than those estimated by changes in biomass standing stock between two consecutive samplings. The difference between both estimates was used to quantify the importance of the processes causing loss of biomass, which were attributable to grazing, exported biomass, and thallus decomposition under anaerobic conditions resulting from extreme self-shading. Maximum chlorophyll content was found in winter, whereas the minimum was in spring. Atomic N:P ratios were generally higher in the algae than in the water. However, the absolute concentrations of tissue N and P were always higher than the critical levels for maximum growth, which suggests that growth was not limited by inorganic N or P availability. The results suggested that the increase in nutrient loading in the river may have triggered the massive development of green algae and that light limitation and temperature stress in summer seem to be the main factors controlling the abundance of Ulva in the estuary. In addition to light availability and thermal stress, the different loss processes may have a decisive role in the dynamics of Ulva biomass.     

Disturbance events affecting phytoplankton biomass,composition and species diversity in a shallow,eutrophic, temperate lake

The seasonal changes in phytoplankton biomass and species diversity in a shallow, eutrophic Danish lake are described and related to different disturbance events acting on the phytoplankton community.Both the spring diatom maximum and the summer bloom of the filamentous blue-green alga, Aphanizomenon flos-aquae (L.) Ralfs, coincided with low values of phytoplankton species diversity and equitability. Diatom collapse was mainly due to internal modifications as nutrient depletion (Si, P) caused by rapid growth of phytoplankton, and increased grazing activity from zooplankton. A large population of Daphnia longispina O.F. Müller in June effectively removed smaller algal competitors, thus favouring the development of a huge summer bloom (140 mm³ l^–1) of Aphanizomenon flos-aquae. Heavy rainfall and storms in late July increased the loss of Apahnizomenon by out-flow and disturbed the stratification of the lake. These events caused a marked decline in phytoplankton biomass but had no effect on species diversity. A second storm period in late August circulated the lake completely and was followed by a rapid increase in phytoplankton diversity, and a change in the phytoplankton community structure from dominance of large, slow-growing K-selected species (Aphanizomenon) to small, fast-growing r-selected species (cryptomonads).     

Grazing impact of microzooplankton on a diatom bloom in a mesocosm as estimated by pigment-specific dilution technique

To investigate the impact of microzooplankton grazing on phytoplankton bloom in coastal waters, an enclosure experiment was conducted in Saanich Inlet, Canada during the summer of 1996. Daily changes in the microzooplankton grazing rate on each phytoplankton group were investigated with the growth rates of each phytoplankton group from the beginning toward the end of bloom using the dilution technique with high-performance liquid chromatography (HPLC). On Day 1 when nitrate and iron were artificially added, chlorophyll a concentration was relatively low (4.3 μg l⁻¹) and 19′-hexanoyloxyfucoxanthin-containing prymnesiophytes were predominant in the chlorophyll biomass. However, both the synthetic rates and concentrations of 19′-hexanoyloxyfucoxanthin declined before bloom, suggesting that 19′-hexanoyloxyfucoxanthin-containing prymnesiophytes weakened. Chlorophyll a concentration peaked at 23 μg l⁻¹ on Day 4 and the bloom consisted of the small chain-forming diatoms Chaetoceros spp. (4 μm in cell diameter). Diatoms were secondary constituents in the chlorophyll biomass at the beginning of the experiment, and the growth rates of diatoms (fucoxanthin) were consistently high (>0.5 d⁻¹) until Day 3. Microzooplankton grazing rates on each phytoplankton group remarkably increased except on alloxanthin-containing cryptophytes after the nutrient enrichments, and peaked with >0.6 d⁻¹ on Day 3, indicating that >45% of the standing stock of each phytoplankton group was removed per day. Both the growth and mortality rates of alloxanthin-containing cryptophytes were relatively high (>1 and >0.5 d⁻¹, respectively) until the bloom, suggesting that a homeostatic mechanism might exist between predators and their prey. Overall, microzooplankton grazing showed a rapid response to the increase in phytoplankton abundance after the nutrient enrichments, and affected the magnitude of the bloom significantly. High grazing activity of microzooplankton contributed to an increase in the abundance of heterotrophic dinoflagellates with 7-24 μm in cell size, the fraction of large-sized (>10 μm) chlorophyll a, and stimulated the growth of larger-sized ciliates after the bloom.     

Distribution and leaf growth of Thalassodendron pachyrhizum den Hartog in Southern Western Australia

The seagrass Thalassodendron pachyrhizum den Hartog grows on limestone reef platforms. Monthly leaf biomass was measured over 2 years and showed a strong seasonal variation with maximum biomass of 500 g m⁻². This seagrass loses all its leaves except for a bud and this characteristic was used to obtain a conservative estimate of productivity by change in standing stock. Leaf growth during the growing season was 6.6 mg Cg⁻¹ day⁻¹. Leaf length frequencies showed that new leaves formed during autumn (March–April). They grew from autumn until spring (November) and began to senesce in summer, followed by leaf fall in late summer (February–March).The growth of rhizome shoots “invading” free substratum space and the growth of new stems was measured for a 300-day period; about 9 leaves were produced in this period.     

Trophic structure and productivity of the lagoonal communities of Tikehau atoll (Tuamotu Archipelago,French Polynesia)

Carbon standing stocks and fluxes were studied in the lagoon of Tikehau atoll (Tuamotu archipelago, French Polynesia), from 1983 to 1988.The average POC concentration (0.7–2000 µm) was 203 mg C m^–3. The suspended living carbon (31.6 mg C m^–3) was made up of bacteria (53%), phytoplankton < 5 µm (14.2%), phytoplankton > 5 µm (14.2%), nanozooplankton 5–35 µm (5.7%), microzooplankton 35–200 µm (4.7%) and mesozooplankton 200–2000 µm (7.9%). The microphytobenthos biomass was 480 mg C m^–2.Suspended detritus (84.4% of the total POC) did not originate from the reef flat but from lagoonal primary productions. Their sedimentation exceeded phytobenthos production.It was estimated that 50% of bacterial biomass was adsorbed on particles. the bacterial biomass dominance was explained by the utilisation of 1) DOC excreted by phytoplankton (44–175 mg C m^–2 day ^–1) and zooplankton (50 mg Cm^–2 day^–1)2) organic compounds produced by solar-induced photochemical reactions 3) coral mucus.50% of the phytoplankton biomass belongs to the < 5 µm fraction. This production (440 mg C m^–2 day^–1) exceeded phytobenthos production (250 mg C m^–2 day^–1) when the whole lagoon was considered.The zooplankton > 35 µm ingested 315 mg C m^–2 day^–1, made up of phytoplankton, nanozooplankton and detritus. Its production was 132 mg C m^–2 day^–1.     

Phytoplankton biomass and primary production in semi-enclosed reef lagoons of the central Great Barrier Reef,Australia

Phytoplankton biomass and primary production rates within semi-enclosed reef lagoons of the central Great Barrier Reef were compared with adjacent shelf waters. Chlorophyll concentrations and surface primary production rates were usually higher in lagoons although seasonal differences were only significant during the summer. Nitrate concentrations were higher in lagoons than in shelf waters year-round. Nano- (<20 m size fraction) or pico-phytoplankton (<2 m size fraction) dominated phytoplankton biomass and production within reef lagoons throughout the year. Net phytoplankton (>10–20 m size fraction), however, were relatively more important in both reef lagoons and open shelf waters during the summer. Biomass-specific production within lagoons (range 41–90 mg C mg chl^–1 day^–1) was high, regardless of season. Lagoonal phytoplankton production (range 0.2–1.6 g C m^–2 day^–1) was directly correlated with standing crop and inversely related to lagoon flushing rates. Phytoplankton blooms develop within GBR reef lagoons during intermittent calm periods when water residence times exceed phytoplankton generation times.     

Nutrient and iron limitation to <Emphasis Type="Italic">Ulva</Emphasis> blooms in a eutrophic coastal lagoon (Sacca di Goro,Italy)

Growth patterns and bloom formation of the green seaweed Ulva rigida were analysed in the eutrophic Sacca di Goro lagoon (Po River Delta, Italy). Variations of standing biomasses and elemental composition of Ulva were analysed through an annual cycle with respect to nitrogen, phosphorus and iron. Growth rates, nutrient and iron uptake and nitrate storage by macroalgal thalli were also assessed with field experiments during the formation of a spring bloom. The control of Ulva growth and the bloom formation depended on multiple factors, especially on nitrogen availability and iron deficiency. In the nitrate rich waters of the Sacca di Goro lagoon, nitrate accumulation in Ulva thalli was inversely related with Fe uptake, indicating an influence of Fe limitation on N acquisition. Since length and magnitude of nitrate luxury uptake are inversely related to the size of the intracellular nitrate pools, in nitrate rich waters the fast growing Ulva may face risk of N-limitation not only when exposed to low N concentrations or at high biomass levels, but also when exposed to pulsed dissolved nitrate concentrations at low iron availability. The potential Fe limitation could be affected by processes controlled by geochemical reactions and by macroalgal growth and decomposition. Both Fe oxidation during the active macroalgal growth and the formation of insoluble FeS and FeS₂ during bloom collapse can result in a drastic decrease of soluble iron. Thus, a potential limitation of Fe to macroalgae can occur, determining positive feedbacks and potentially controlling the extent of bloom development and persistence.