Biogeochemistry and microbial community composition in sea ice and underlying seawater off East Antarctica during early spring

Pack ice, brines and seawaters were sampled in October 2003 in the East Antarctic sector to investigate the structure of the microbial communities (algae, bacteria and protozoa) in relation to the associated physico-chemical conditions (ice structure, temperature, salinity, inorganic nutrients, chlorophyll a and organic matter). Ice cover ranged between 0.3 and 0.8 m, composed of granular and columnar ice. The brine volume fractions sharply increased above −4°C in the bottom ice, coinciding with an important increase of algal biomass (up to 3.9 mg C l⁻¹), suggesting a control of the algae growth by the space availability at that period of time. Large accumulation of NH₄ ⁺ and PO₄ ³⁻ was observed in the bottom ice. The high pool of organic matter, especially of transparent exopolymeric particles, likely led to nutrients retention and limitation of the protozoa grazing pressure, inducing therefore an algal accumulation. In contrast, the heterotrophs dominated in the underlying seawaters.     

The diversity,abundance and fate of ice algae and phytoplankton in the Bering Sea

Despite being an essential part of the marine food web during periods of ice cover, sea ice algae have not been studied in any detail in the Bering Sea. In this study, we investigated the diversity, abundance and ultimate fate of ice algae in the Bering Sea using sea ice, water and sub-ice sediment trap samples collected during two spring periods in 2008 and 2009: ice growth (March–mid-April) and ice melt (mid-April–May). The total ice algal species inventory included 68 species, dominated by typical Arctic ice algal diatom taxa. Only three species were determined from the water samples; we interpret the strong overlap in species as seeding of algal cells from the sea ice. Algal abundances in the ice exceeded 10⁷ cells l^?1 in the bottom 2-cm layer and were on average three orders of magnitude higher than in the water column. The vertical flux of algal cells beneath the ice during the period of ice melt (>10⁸ cells m^?2 day^?1) exceeded export during the ice growth period by one order of magnitude; the vertical flux during both periods can only be sustained by the release of algae from the ice. Differences in the relative species proportions of algae among sample types indicated that the fate of the released ice algae was species specific, with some taxa contributing to seeding in the water column, while other taxa were preferentially exported.     

Characteristics of two distinct high-light acclimated algal communities during advanced stages of sea ice melt

Biological characteristics of ice-associated algal communities were studied in Darnley Bay (western Canadian Arctic) during a 2-week period in July 2008 when the landfast ice cover had reached an advanced stage of melt. We found two distinct and separate algal communities: (1) an interior ice community confined to brine channel networks beneath white ice covers; and (2) an ice melt water community in the brackish waters of both surface melt ponds and the layer immediately below the ice cover. Both communities reached maximum chlorophyll?a concentrations of about 2.5?mg?m^?3, but with diatoms dominating the interior ice while flagellates dominated the melt water community. The microflora of each community was diverse, containing both unique and shared algal species, the latter suggesting an initial seeding of the ice melt water by the bottom ice community. Absorption characteristics of the algae indicated the presence of mycosporine-like amino acids (MAAs) and carotenoid pigments as a photoprotective strategy against being confined to high-light near-surface layers. Although likely not contributing substantially to total annual primary production, these ice-associated communities may play an important ecological role in the Arctic marine ecosystem, supplying an accessible and stable food source to higher trophic levels during the period of ice melt.     

MICROALGAL LIGHT-HARVESTING IN EXTREME LOW-LIGHT ENVIRONMENTS IN MCMURDO SOUND, ANTARCTICA1

Microalgal pigment composition, photosynthetic characteristics, single-cell absorption efficiency (Qa_(λ)) spectra, and fluorescence-excitation (FE) spectra were determined for platelet ice and benthic communities underlying fast ice in Mc Murdo Sound, Antarctica, during austral spring 1988. Measurements of spectral irradiance (E_(λ)) and photosynthetically active radiation (PAR) as well as samples for particulate absorption measurements were taken directly under the congelation ice, within the platelet layer, as profiles vertically through the water column, and at the benihic surface. Light attenuation by.sea ice, algal pigments, and particulates reduced PAR reaching the platelet ice layer to 3%(9–33 fimol photons m-²-^?s-¹) of surface values and narrowed its spectral distribution to a band between 400 and 580 nm. Attenuation by the water column further reduced PAR reaching the sea floor (28–m depth) to 0.05% of surface levels (< 1 μmol photons m-² s-¹), with a spectral distribution dominated by 470–580–nm wavelengths. The photoadaptive index (I) for platelet ice algae (5.9–12.6 μmol photons m-^2.s-¹) was similar to ambient PAR, indicating that algae had acclimated to their light environment (i.e. the algae were light-replete). Maximum Qa_(λ) at the blue absorption peak (440 nm) was 0.63, and enhanced absorption was observed from 460–500 nm and was consistent with observed high cellular chlorophyll (chi) c:chl a and fucoxanthin: chl a molar ratios (0.4 and 1.2, respectively). Benthic algae were light-limited despite the maintenance of very low I_k values (4–11 μmol photons.m-^2.s-¹). Extremely high fucoxanthin: chi a ratios (1.6) in benthic algae produced enhanced green light absorption, resulting in a high degree of complementation between algal absorption and ambient spectral irradiance. Qa_(λ) values for benthic algae were maximal (0.9) between 400 and 510 nm but remained >0.35 even at absorption minima. Strong spectral flattening, a characteristic of intense pigment packaging, was also apparent in the Qa_(λ) spectra for benthic algae. FE and Qa_(λ) spectra were similar in shape for platelet ice algae, indicating that the efficiency at which absorbed energy was transferred to photosystem II (PSII) was independent of wavelength. Fluorescence emission by benthic algae was greatest for the 500–560–nm excitation wavelengths, suggesting that most energy absorbed by accessory pigments was transferred to PSII. These results suggest that under ice algae employ complementary pigmentation and maximize absorption efficiency as adaptive strategies to low-light stress. Regulating the distribution of absorbed energy between PSI and PSII may be an adaptive response to the restricted spectral distribution of irradiance.     

Effects of snow removal and algal photoacclimation on growth and export of ice algae

Net growth of ice algae in response to changes in overlying snow cover was studied after manipulating snow thickness on land-fast, Arctic sea ice. Parallel laboratory experiments measured the effect of changing irradiance on growth rate of the ice diatom, Nitzschia frigida. After complete removal of thick snow (≥9 cm), in situ ice algae biomass declined (over 7–12 days), while removal of thin snow layers (4–5 cm), or partial snow removal, increased net algal growth. Ice bottom ablation sometimes followed snow removal, but did not always result in net loss of algae. Similarly, in laboratory experiments, small increases in irradiance increased algal growth rate, while greater light shifts suppressed growth for 3–6 days. However, N. frigida could acclimate to relatively high irradiance (110 μmol photons m² s⁻¹). The results suggest that algal loss following removal of a thick snow layer was due to the combination of photoinhibition and bottom ablation. The smaller relative increase in irradiance after removal of thin or partial snow layers allowed algae to maintain high specific-growth rates that compensated for loss from physical mechanisms. Thus, the response of ice algae to snow loss depends both on the amount of change in snow depth and algal photophysiology. The complex response of ice algae growth and export loss to frequently changing snow fields may contribute to horizontal and temporal patchiness of ecologically and biogeochemically important variables in sea ice and should be considered in predictions of how climate change will affect Arctic marine ecosystems.     

THE SHORT‐TERM EFFECT OF IRRADIANCE ON THE PHOTOSYNTHETIC PROPERTIES OF ANTARCTIC FAST‐ICE MICROALGAL COMMUNITIES1

Although sea‐ice represents a harsh physicochemical environment with steep gradients in temperature, light, and salinity, diverse microbial communities are present within the ice matrix. We describe here the photosynthetic responses of sea‐ice microalgae to varying irradiances. Rapid light curves (RLCs) were generated using pulse amplitude fluorometry and used to derive photosynthetic yield (Φ_PSII), photosynthetic efficiency (α), and the irradiance (E_k) at which relative electron transport rate (rETR) saturates. Surface brine algae from near the surface and bottom‐ice algae were exposed to a range of irradiances from 7 to 262 μmol photons · m^?2 · s^?1. In surface brine algae, Φ_PSII and α remained constant at all irradiances, and rETR_max peaked at 151 μmol photons · m^?2 · s^?1, indicating these algae are well acclimated to the irradiances to which they are normally exposed. In contrast, Φ_PSII, α, and rETR_max in bottom‐ice algae reduced when exposed to irradiances >26 μmol photons · m^?2 · s^?1, indicating a high degree of shade acclimation. In addition, the previous light history had no significant effect on the photosynthetic capacity of bottom‐ice algae whether cells were gradually exposed to target irradiances over a 12 h period or were exposed immediately (light shocked). These findings indicate that bottom‐ice algae are photoinhibited in a dose‐dependent manner, while surface brine algae tolerate higher irradiances. Our study shows that sea‐ice algae are able to adjust to changes in irradiance rapidly, and this ability to acclimate may facilitate survival and subsequent long‐term acclimation to the postmelt light regime of the Southern Ocean.     

PHOTOPHYSIOLOGICAL EVIDENCE OF NUTRIENT LIMITATION OF PLATELET ICE ALGAE IN MCMURDO SOUND, ANTARCTICA

Seasonally changing photophysiological and biochemical characteristics of sea ice microalgae are interpreted with respect to light availability and measurements of nutrient concentration made at high vertical resolution (12.5 cm) during a dense bloom in the platelet ice layer of McMurdo Sound during a 6-week study in austral spring of 1989. Platelet ice algae remained highly shade adapted throughout the spring as shown by their low photoadaptive index (E_k, 3.7–8.4 μmol photons·m⁻²·s⁻¹), low mean specific absorption coefficient (<0.009 m² mg⁻¹ Chl a), high optical cross-sectional area of photosystem II (σ_PSII, 3.0–8.2), and high molar ratio of fucoxanthin:chlorophyll a (mean = 1.62 ± 0.15 SD). Between 24 October and 8 November, the algae exhibited a photoacclimative response that was marked by a 30% decrease in photosynthetic efficiency (α^B), a 75% decrease in maximum photosynthetic rate (P^B/_m), and a 60% increase in σ_PSII. The photochemical conversion efficiency at photosystem II (F_v/F_m= ca. 0.5) and the quantum yield of photosynthesis (Ø_C= 0.062– 0.078 mol C mol⁻¹ photons) were ca. 80% of their maximal values. After 8 November, changes in algal photophysiology and biochemistry, which were inconsistent with a photoacclimation response, suggest that the platelet ice algae near the platelet/congelation ice interface became increasingly nutrient limited. The number of pennate diatoms increased threefold to 150 × 10⁹ cells m⁻³ between 8 and 14 November, then remained unchanged throughout the remainder of the field season. Following the increase in cell number, F_v/F_m, Ø_C, and C:Chla decreased by >40%, σ_PSII increased by 70%; and the biochemical ratios C:N and C:Si increased 25%–30%. Nutrient depletion was apparent from the high-resolution vertical profiles, but nutrient concentrations limiting algal growth were not observed. However, nutrient concentrations at the likely site of nutrient limitation near the platelet/congelation ice interface were not measured, indicating that higher resolution sampling is necessary to fully characterize this highly variable habitat.     

COMPARISON OF IN SITU PHOTOSYNTHETIC ACTIVITY OF EPIPHYTIC,EPIPELIC AND PLANKTONIC ALGAL COMMUNITIES IN AN OLIGOTROPHIC LAKE,SOUTHERN FINLAND1

The photosynthetic activity of different algal communities at the outer edge of an Equisetum fluviatile L. stand in an oligotrophic lake (Pääjärvi, in southern Finland) was investigated. Production by the algal communities was measured simultaneously using a modified ¹⁴C-method, and the results were related to the volume of algae and the available irradiance. The relative production rate (P/B quotient) of phytoplankton was ca. 3 × that of epiphyton and ca. 20 × that of epipelon. Epiphyton productivity remained almost constant although the algal volume varied greatly, suggesting that the surface layer of the algal community was mainly responsible for the photosynthetic activity. In the littoral area (at 1 m depth) primary production/m² of lake surface by phytoplankton, epiphyton and epipelon was similar but in the littoriprofundal area (2–4 m) phytoplankton production was twice that of epipelon. Primary productivity of epiphyton and epipelon/m² of substratum was about equal to phytoplankton productivity/m³ of water at the same irradiance. This relation provided a means of estimating the relative contributions of the different algal communities to the total algal production in the lake.     

PARTICULATE DIMETHYLSULFOXIDE IN ARCTIC SEA-ICE ALGAL COMMUNITIES: THE CRYOPROTECTANT HYPOTHESIS REVISITED

The particulate-phase concentrations of dimethyl sulfoxide (DMSO_p) and dimethylsulfoniopropionate (DMSP_p) in sea-ice algal communities from the North Water, northern Baffin Bay, were examined from April to June 1998. The concentrations of these compounds were measured in the bottom 2 cm of the ice at 36 locations throughout this region and are compared with results from water-column samples collected for a complementary study. In general, levels of DMSP_p (8.66–987 nmol·L ^{− 1}, average 126 nmol·L ^{− 1}) in sea-ice algal communities were slightly less than those found in bottom sea-ice algal communities from other polar locations but greater than those found in phytoplankton in other polar environments or at more temperate latitudes. Furthermore, DMSP_p :chl a ratios (0.02–14.8 nmol·μg ^{− 1}, average 1.91 nmol·μg ^{− 1}) in the sea-ice algal community were slightly less than those found in other polar environments. DMSO_p was measured for the first time in sea-ice algal communities. DMSO_p concentrations varied from 1.35 to 102 nmol·L ^{− 1} (average 13.7 nmol·L ^{− 1}). DMSO_p:chl a ratios varied from 0.01 to 4.5 nmol·μg ^{− 1} (average 0.22 nmol·μg ^{− 1}) and were significantly lower than the DMSP_p:chl a ratios observed in this study. It has been hypothesized that DMSO can act as a cryoprotector in phytoplankton cells. However, the low concentrations of DMSO observed in the ice algae during this study indicate that intracellular concentrations of DMSO are unlikely to have a significant influence on the freezing point depression of intracellular fluids.     

Experimental Evidence on Nutrient and Substrate Limitation of Baltic Sea sea-ice Algae and Bacteria

The effect of nutrient limitation on Baltic Sea ice algae, and substrate and nutrient limitation on ice bacteria, was studied in a series of in situ -experiments conducted during the winter of 2002 in northern Baltic Sea. Community level changes in algal biomass (chlorophyll a) and productivity, and bacterial thymidine and leucine incorporation were followed for one week after the addition of nutrient and/or organic carbon rich filtered seawater to the experimental units. The results showed the major contribution of snow cover to the algal responses during the beginning of the ice-covered season. Algal communities were able to grow even in January if no snow was present. Nutrient addition did occasionally have an effect on algal biomass and productivity in the ice. Surprisingly, seeding effect from the ice to the underlying water was negatively affected by the nutrient availability in March. Bacterial limitation varied between nutrient (phosphorus) and substrate limitations. The results showed, that limitation in both algal and bacterial communities changed periodically in the northern Baltic Sea ice.     

THE EFFECTS OF ULTRAVIOLET‐B RADIATION ON ANTARCTIC SEA‐ICE ALGAE1

The impacts of ultraviolet‐B radiation (UVB) on polar sea‐ice algal communities have not yet been demonstrated. We assess the impacts of UV on these communities using both laboratory experiments on algal isolates and by modification of the in situ spectral distribution of the under‐ice irradiance. In the latter experiment, filters were attached to the upper surface of the ice so that the algae were exposed in situ to treatments of ambient levels of PAR and UV radiation, ambient radiation minus UVB, and ambient radiation minus all UV. After 16 d, significant increases in chl a and cell numbers were recorded for all treatments, but there were no significant differences among the different treatments. Bottom‐ice algae exposed in vitro were considerably less tolerant to UVB than those in situ, but this tolerance improved when algae were retained within a solid block of ice. In addition, algae extracted from brine channels in the upper meter of sea ice and exposed to PAR and UVB in the laboratory were much more tolerant of high UVB doses than were any bottom‐ice isolates. This finding indicates that brine algae may be better adapted to high PAR and UVB than are bottom‐ice algae. The data indicate that the impact of increased levels of UVB resulting from springtime ozone depletion on Antarctic bottom‐ice communities is likely to be minimal. These algae are likely protected by strong UVB attenuation by the overlying ice and snow, by other inorganic and organic substances in the ice matrix, and by algal cells closer to the surface.     

The sub-ice algal community in the Chukchi sea: large- and small-scale patterns of abundance based on images from a remotely operated vehicle

We examined the sub-ice algal community in the Chukchi Sea during June 1998 using a remotely operated vehicle (ROV). Ice algae were observed on the under-ice surface at all ten stations (from 70°29′N to 72°26′N; 162°00′W to 153°56′W) and varied in abundance and distribution from small aggregations limited to depressions in the ice to nets, curtains and strands of Melosira. There was no relationship between percent cover of sub-ice algae and physical factors at the kilometer scale, but at the scale of individual ice floes the percent cover of sub-ice algae was positively correlated with distance from the floe edge and negatively correlated with snow depth. A significant positive relationship between the concentration of sediment pigments and percent cover of sub-ice could indicate a coupling between ice algal and benthic systems. Pieces of ice algae that appeared to be Melosira were observed on the seafloor to a depth of over 100 m and cells or spores of obligate ice algal taxa were collected from sediments from 44-m to 1,000-m deep. The large biomass of sub-ice algae observed at many stations in the Chukchi Sea and the presence of ice algae on the seafloor indicates that the distribution and abundance of sub-ice algae needs to be understood if we are to evaluate the role of ice algae in the Arctic marine ecosystem.     

SEASONAL SUCCESSION OF ALGAL PERIPHYTON FROM A WASTEWATER TREATMENT FACILITY1

Attached algal populations were sampled at weekly or biweekly to characterize successional changes in the secondary clarifiers of a wastewater treatment plant. Three communities were compared from areas of slow, medium and rapid current velocities. In general, the algae resembled those reported for other hypereutrophic flowing water. Of the twenty-three algae recorded, Stigeoclonium, Oedogonium, Oscillatoria, Lyngbya, and Pleurocapsa were dominant at some point in the 15 month sampling period. Nutrient concentrations were consistently high (N = 1.1–21.4 mg·L⁻¹; P = 0.1–10.4 mg·L⁻¹); therefore, changes in temporal distribution of algae were probably dependent on seasonal changes in light and temperature. Colonization of artificial substrates was also observed. Small unicellular algae were the first autotrophs to attach and these were followed by larger filamentous forms.     

Dependency of Antarctic zooplankton species on ice algae‐produced carbon suggests a sea ice‐driven pelagic ecosystem during winter

How the abundant pelagic life of the Southern Ocean survives winter darkness, when the sea is covered by pack ice and phytoplankton production is nearly zero, is poorly understood. Ice‐associated (“sympagic”) microalgae could serve as a high‐quality carbon source during winter, but their significance in the food web is so far unquantified. To better understand the importance of ice algae‐produced carbon for the overwintering of Antarctic organisms, we investigated fatty acid (FA) and stable isotope compositions of 10 zooplankton species, and their potential sympagic and pelagic carbon sources. FA‐specific carbon stable isotope compositions were used in stable isotope mixing models to quantify the contribution of ice algae‐produced carbon (α_Ice) to the body carbon of each species. Mean α_Ice estimates ranged from 4% to 67%, with large variations between species and depending on the FA used for the modelling. Integrating the α_Ice estimates from all models, the sympagic amphipod Eusirus laticarpus was the most dependent on ice algal carbon (α_Ice: 54%–67%), and the salp Salpa thompsoni showed the least dependency on ice algal carbon (α_Ice: 8%–40%). Differences in α_Ice estimates between FAs associated with short‐term vs. long‐term lipid pools suggested an increasing importance of ice algal carbon for many species as the winter season progressed. In the abundant winter‐active copepod Calanus propinquus, mean α_Ice reached more than 50% in late winter. The trophic carbon flux from ice algae into this copepod was between 3 and 5 mg C m^?2 day^?1. This indicates that copepods and other ice‐dependent zooplankton species transfer significant amounts of carbon from ice algae into the pelagic system, where it fuels the food web, the biological carbon pump and elemental cycling. Understanding the role of ice algae‐produced carbon in these processes will be the key to predictions of the impact of future sea ice decline on Antarctic ecosystem functioning.     

Development of ice biota in a temperate sea area (Gulf of Bothnia)

A study of sea ice biota was carried out in the Gulf of Bothnia (northern Baltic Sea) during the winter of 1989–1990. Samples (ice cores) were taken at a coastal station at regular time intervals during the ice season. Chlorophyll a concentration, algal species distribution, bacterial numbers, and primary and bacterial production were measured. Colonization of the ice began in January when daylight was low. As the available light increased, the algae started to grow exponentially. The vertical chlorophyll a distribution changed and algal species composition and biomass changed during the season. During the initial and middle phase of colonization, ice-specific diatoms, Nitzschia frigida and Navicula pelagica, dominated the algal biomass. Nutrients (PO₄ ^3– and NO_3j) were found to be depleted during the time of algal exponential growth. The maximum algal biomass exceeded 800 g C 1^–1. The primary production supplied food for heterotrophic organisms. The presence of heterotrophic organisms of different trophic levels (bacteria, flagellates, ciliates and rotifers) indicated an active microbial food web.     

Linking ice structure and microscale variability of algal biomass in Arctic first-year sea ice using an in situ photographic technique

Microscale photographs were taken of the ice bottom to examine linkages of algal chlorophyll a (chl a) biomass distribution with bottom ice features in thick Arctic first-year sea ice during a spring field program which took place from May 5 to 21, 2003. The photographic technique developed in this paper has resulted in the first in situ observations of microscale variability in bottom ice algae distribution in Arctic first-year sea ice in relation to ice morphology. Observations of brine channel diameter (1.65–2.68 mm) and number density (5.33–10.35 per 100 cm²) showed that the number of these channels at the bottom of thick first-year sea ice may be greater than previously measured on extracted ice samples. A variogram analysis showed that over areas of low chl a biomass (≤20.7 mg chl a m⁻²), patchiness in bottom ice chl a biomass was at the scale of brine layer spacing and small brine channels (∼1–3 mm). Over areas of high chl a biomass (≥34.6 mg chl a m⁻²), patchiness in biomass was related to the spacing of larger brine channels on the ice bottom (∼10–26 mm). Brine layers and channels are thought to provide microscale maxima of light, nutrient replenishment and space availability which would explain the small scale patchiness over areas of low algal biomass. However, ice melt and erosion near brine channels may play a more important role in areas with high algal biomass and low snow cover.     

Changes in Phytoplankton Biomass and Nutrient Quantities in Sea Ice as Responses to Light/Dark Manipulations during different Phases of the Baltic Winter 2003

The response of Baltic Sea ice communities to changing light climate was studied in three subsequent 3 week in situ experiments on the SW coast of Finland. The investigation covered three different winter periods, short day with low solar angles leading to limited light in the ice, late winter with deep snow cover and early spring with melting snow and increasing light availability. The experimental setup consisted of transparent (no snow) and completely darkened (heavy snow cover) plexiglass tubes in which the ice cores were incubated in situ from 1 to 2 weeks. Changes in the concentrations of inorganic nutrients (NO₃⁻-–N, PO₄³⁻-–P, SiO₄⁻-–Si) and chlorophyll-a concentration in the phytoplankton community composition were recorded as responses to different light manipulations. Changes in inner ice light intensity in untreated ice as well as the temperature both in air and ice were recorded over the entire study period. Increased irradiance in late winter/early spring and during meltdown affected the chlorophyll-a amount in the sea ice. During these periods the phytoplankton community in the top layers decreased possibly as a consequence of photo-acclimation. Closer to the bottom of the ice, however, the increased inner ice light intensity induced algal growth. Complete exclusion of light stopped the algal growth in the whole ice column. Darkening the ice cores also slowed down the ice melting opposite to accelerated melting caused by increased light. The significant differences found in nutrient concentrations between the light and dark treatments were mostly explicable by changes in algal biomass. No obvious changes were observed in the phytoplankton community composition due to light manipulation, diatoms and heterotrophic flagellates dominating throughout the study period.     

FLUORESCENCE INDUCTION AND PHOTOSYNTHETIC RESPONSES OF ARCTIC ICE ALGAE TO SAMPLE TREATMENT AND SALINITY1

The measurement of Photosynthetic rates of algae growing on the undersurface of 1. 7 m thick ice in the Canadian Arctic (Resolute Passage. N.W.T.) presents several problems. During the preparation of samples for physiological measurements, the ice algae may he exposed to salinity and temperature shocks. Fluorescence induction (the rise in in vivo Chl a fluorescence intensity during a period of millineconds) and photosynthesis-irradiance (PI) experiments examined the potential effects of salinity and temperature on the physiology of ice algae. Experimental suspensions were routinely prepared by scraping one part ire crystals (11–14%₀ salinity) and attached algae from the bottom ice into four parts filtered seawater (32%₀ salinity). giving a final salinity of 28–31%₀. Post-dilution of melted ice scrapings with seawater suppressed photosynthetic ¹⁴C-fixation and decreased A_DCMU (the area above the fluorescence induction curve measured in the presence of the inhibitor DCMC: an estimate of photosynthetic capacity) by a factor of 3–16. due to the low salinity of the melted ice scrapings. Fluorescence induction and PI experiments showed that the ice algae had a salinity optimum near 30%₀, close to the ambient seawater salinity, Experiments in which the Chl a concentration was manipulated showed that A_DCMU, P^a_m (Chl a-normalized rate of photosynthesis at light saturation), and a (photosynthetic efficiency) declined with increasing Chl a concentration. Ice algae tolerated heating (l.5°C-min^-1) up to 17° C, above which A_DCMU’decreased with sample temperature.     

Evidence of coprophagy in freshwater zooplankton

1. Vertical transport of nutrients in sedimenting faecal material is greatly reduced by coprophageous organisms. Unfortunately, nearly all work on faecal production, sedimentation and coprophagy has dealt with copepods in marine ecosystems. Here, we report the first evidence of coprophagy in freshwater zooplankton from oligotrophic and eutrophic lakes. We used ¹⁴C‐labelled algae and faecal material to estimate the rates of algal clearance and coprophagy. 2. Measured feeding rates per individual on faecal material were similar (Daphnia pulex, D. rosea, Leptodiaptomus tyrelli) or even higher (D. lumholtzi) than filtering rates on phytoplankton. This finding does not necessarily implicate active selection of faeces over algae because: (i) we did not use the same food concentrations for faeces and algae, and (ii) grazers of slightly different sizes were used in each test. 3. Weight‐specific clearance rates of L. tyrelli and Holopedium gibberum on faecal matter (0.084–0.089 mL μg^?1 h^?1) were higher than in the daphniids (0.026 mL μg^?1 h^?1). 4. The data indicate that coprophagy in freshwater ecosystems is an important mechanism of nutrient recycling, and this process should be taken into account when studying nutrient fluxes within lakes and reservoirs.     

Molluscan grazing and macroalgal zonation on a rocky intertidal platform at Perth,Western Australia

Abstract Intertidal limestone platforms off Perth show a characteristic pattern of algal zonation, with dense macroalgal beds nearshore bounded by a ‘barren zone’ along the seaward edge. Abalone (Haliotis roei) and several species of limpets and chitons are abundant in the barren zone, which is generally devoid of non-coralline macroalgae. The relative importance of abalone versus limpets and chitons in limiting macroalgal abundance in the barren zone was evaluated by manipulating the presence of each group in a factorial experiment. Percentage algal cover was measured photographically in 0.25m² plots at 1–2 month intervals for 9 months. Mean algal cover (mainly the foliose green alga, Ulva rigida) was highest in plots where all grazers were excluded (77–99%), intermediate where only limpets and chitons were excluded (37–85%), and lowest where only abalone were excluded (4–30%) or where no grazers were excluded (2–19%). The effect of limpets and chitons accounted for 55–89% of the variance in total algal cover, whereas the effect of abalone generally accounted for <10% of the variance. Similar results were obtained in terms of the biomass of Ulva rigida at the end of the experiment. Haliotis roei are relatively large and sedentary herbivores, feeding mainly on drift algae. Their effect on algal abundance was mediated both through pre-emption of space, which might otherwise be colonized by algae, and by grazing around their home scar. Limpets and chitons are smaller than abalone, but were much more abundant. Intensive grazing of the reef surface by limpets and chitons precluded the establishment of non-coralline macroalgae, even where abalone were absent.