Salps in the Lazarev Sea, Southern Ocean: II. Biochemical composition and potential prey value

Two species of salps, Salpa thompsoni and Ihlea racovitzai, were sampled during three cruises to the Lazarev Sea, Southern Ocean, in summer (December–January) 2005/2006, Autumn (April–May) 2004 and Winter (July–August) 2006. Dry weight, carbon, nitrogen, protein, lipid and carbohydrate contents were measured to characterize the potential value of salps as a food source for predators in the Antarctic ecosystem. Biochemical composition measurements showed that despite having a high percentage of water (~94% of wet weight), both species had relatively high carbon and protein contents in their remaining dry weight (DW). In particular I. racovitzai showed high carbon (up to 22% of DW) and protein (up to 32% of DW) values during all seasons sampled, compared to lower values for S. thompsoni (carbon content only about 15% of the DW, protein content about 10% of the DW). At the same time, carbohydrates (CH) and lipids (Lip) only accounted for a small portion of salp DW in both species (1.4% CH and 3.6% Lip for I. racovitzai; 2.1% CH and 2.9% Lip for S. thompsoni). There was little variability in the biochemical composition of either salp species between the seasons sampled. Both biochemical composition and life cycle characteristics suggest that Antarctic salps, especially I. racovitzai, may be important prey items for both cold and warm-blooded predators in an environment where food is often very scarce.     

Metabolism and elemental composition of four oncaeid copepods in the western subarctic Pacific

Respiration rates and elemental composition (carbon and nitrogen) were determined for four dominant oncaeid copepods (Triconia borealis, Triconia canadensis, Oncaea grossa and Oncaea parila) from 0–1,000 m depth in the western subarctic Pacific. Across the four species of which dry weight (DW) varied from 2.0 to 32 μg, respiration rates measured at in situ temperature (3°C) increased with DW, ranging from 0.84 to 7.4 nl O₂ individual⁻¹ h⁻¹. Carbon (C) and nitrogen (N) composition of the four oncaeid species ranged from 49–57% of DW and 7.0–10.3% of DW, respectively, and the resultant C:N ratios were 4.8–8.3. The high C contents and C:N ratios were reflected by large accumulation of lipids in their body. Specific respiration rates (SR, a fraction of body C respired per day) ranged between 0.5 and 1.3% day⁻¹. Respiration rates adjusted to a body size of 1 mg body N (i.e. adjusted metabolic rates, AMR) of the four oncaeid species [0.6–1.1 μl O₂ (mg body N)^−0.8 h⁻¹ at 3°C] were significantly lower than those (1.7–5.1) reported in the literature for oithonid and calanoid copepods at the same temperature. The present results indicate that lower metabolic expenditure due to less active swimming (pseudopelagic life mode) together with rich energy reserve in the body (as lipids) are the characters of oncaeid copepods inhabiting in the epi- and mesopelagic zones of this region.     

Proximate and elemental composition and energy content of mesopelagic crustaceans from the Eastern Gulf of Mexico

The proximate and elemental chemical compositions of 25 species of pelagic decapod and mysid crustaceans collected from the eastern Gulf of Mexico (27°N; 86°W, 1984 to 1989) was examined. Water level ranged from 63 to 95% and increased slightly with species' increased depth of occurrence. Protein levels were generally high (1.5 to 18.3% wet wt, WW; 27.6 to 62.4% ash-free dry wt, AFDW) and comprised the primary organic component in the majority of species. Protein, both as % WW and % AFDW, decreased with increased depth of occurrence. In contrast to protein, lipid levels were low (0.5 to 8.9% WW; 5.7 to 60.9% AFDW), and increased with increased depth of occurrence. Carbon and nitrogen best mirrored measured lipid and protein levels when considered as non-protein carbon and non-chitin nitrogen, respectively. C:N ratios increased with increased depth, consistent with changes in protein and lipid with depth. Because of their compositional attributes, resident Gulf of Mexico species have a low total wet weight energy content relative to species from more productive regions. Energy content showed no significant trend with depth. Vertical migration patterns were distinctly different between shallow-and deep-living gulf species and these differences were largely responsible for the relationships observed between composition and depth. In migrating species, the protein and nitrogen content were higher, the lipid and carbon contents and C:N ratio lower, than in non-migrating species. Three deep-living species of the genus Acanthephyra were found to be compositionally atypical, resembling shallow, migrating types rather than other deep-living, non-migratory species.     

Developmental changes in body weight and elemental composition of laboratory-reared larvae of the red frog crab,Ranina ranina (Decapoda: Brachyura)

Ranina ranina larvae were reared at 29°C from hatching to the megalopa stage to measure daily changes in body weight, water content and elemental composition. Energy, estimated from carbon content, was also examined. The water content was 85 to 92% of body weight immediately after ecdysis but decreased with days after ecdysis. Gains in body weight, carbon, nitrogen, hydrogen and energy during each instar ranged from 52 to 245% and increased with instar after instar II (body weight and carbon), instar III (hydrogen and energy), and instar IV (nitrogen). Cumulative gains of these elements from hatching to 2 d before metamorphosis into megalopa ranged from 11 567% (carbon) to 12 209% (energy). Most cumulative gains (57 to 59%) in elemental composition were contributed by instar VII. Carbon, nitrogen and hydrogen content in body weight decreased to a minimum on the day of ecdysis and increased on the subsequent days. C:N ratios after instar IV were lowest on the day after ecdysis and reached a plateau by the second day. Energy, estimated as J mg^-1 dry weight (DW), decreased with instar and within a molt cycle, and was at a minimum on the day after ecdysis. Gains in elemental composition could be described by an exponential function of days after hatching and by a quadratic function in each instar.     

The tunicate Salpa thompsoni ecology in the Southern Ocean. I. Distribution, biomass, demography and feeding ecophysiology

Distribution, density, and feeding dynamics of the pelagic tunicate Salpa thompsoni have been investigated during the expedition ANTARKTIS XVIII/5b to the Eastern Bellingshausen Sea on board RV Polarstern in April 2001. This expedition was the German contribution to the field campaign of the Southern Ocean Global Ocean Ecosystems Dynamics Study (SO-GLOBEC). Salps were found at 31% of all RMT-8 and Bongo stations. Their densities in the RMT-8 samples were low and did not exceed 4.8 ind m⁻² and 7.4 mg C m⁻². However, maximum salp densities sampled with the Bongo net reached 56 ind m⁻² and 341 mg C m⁻². A bimodal salp length frequency distribution was recorded over the shelf, and suggested two recent budding events. This was also confirmed by the developmental stage composition of solitary forms. Ingestion rates of aggregate forms increased from 2.8 to 13.9 μg (pig) ind⁻¹ day⁻¹ or from 0.25 to 2.38 mg C ind⁻¹ day⁻¹ in salps from 10 to 40 mm oral-atrial length, accounting for 25–75% of body carbon per day. Faecal pellet production rates were on average 0.08 pellet ind⁻¹ h⁻¹ with a pronounced diel pattern. Daily individual egestion rates in 13 and 30 mm aggregates ranged from 0.6 to 4.8 μg (pig) day⁻¹ or from 164 to 239 μg C day⁻¹. Assimilation efficiency ranged from 73 to 90% and from 65 to 76% in 13 and 30 mm aggregates, respectively. S. thompsoni exhibited similar ingestion and egestion rates previously estimated for low Antarctic (~50°S) habitats. It has been suggested that the salp population was able to develop in the Eastern Bellingshausen Sea due to an intrusion into the area of the warm Upper Circumpolar Deep Water     

Defecation by Salpa thompsoni and its contribution to vertical flux in the Southern Ocean

Measurements of the defecation rate of Salpa thompsoni were made at several stations during two cruises west of the Antarctic Peninsula in 2004 and 2006. Rates were quantified in terms of number of pellets, pigment, carbon and nitrogen for a wide size range of both aggregate and solitary salps. Measured defecation rates were constant over several hours when salps were held at near-surface conditions from which they had been collected. The defecation rate per salp increased with both salp size and the ambient level of particulate organic matter (POM) in the upper water column. The weight-specific defecation rate ranged between 0.5 and 6% day⁻¹ of salp body carbon, depending on the concentration of available particulate matter in the water. Carbon defecation rates were applied to biomass estimates of S. thompsoni to calculate daily carbon defecation rates for the populations sampled during the two cruises. Dense salp populations of over 400 mg C m⁻² were calculated to produce about 20 mg C m⁻² day⁻¹, comparable to other major sources of vertical flux of organic material in the Southern Ocean. Measured sinking rates for salp fecal pellets indicated that the majority of this organic material could reach deep sediments within a few days, providing a fast and direct pathway for carbon to the deep ocean.     

Salps in the Lazarev Sea,Southern Ocean: I. Feeding dynamics

Feeding dynamics of the Antarctic salps Ihlea racovitzai and Salpa thompsoni were studied in the Lazarev Sea in fall 2004, summer 2005–2006 and winter 2006. Pigment concentrations in the guts of both species were positively correlated with ambient surface chlorophyll a (chl a). No evidence was found for salp clogging even at dense surface concentrations of up to 7 μg chl a L⁻¹. However, gut pigment concentrations had a lower range than ambient pigment concentrations, suggesting that salps increased retention times of ingested material in low-food environments. For medium-sized I. racovitzai and S. thompsoni, estimated individual daily rations reached 7–10 and >100% of body carbon in winter and summer, respectively. Daily respiratory needs of I. racovitzai and S. thompsoni accounted for 28 and 22% of daily carbon assimilation based on pigment ingestion rates in winter, and for 2 and 1% in summer, respectively. The grazing impact of the salp populations on the phytoplankton standing stock was negligible during all seasons due to generally low salp densities. Fatty acid trophic biomarkers in the salps suggest high year-round contributions of flagellates and modest contributions of diatoms to the salp’s diet. These markers showed low seasonal variability for I. racovitzai. The more pronounced seasonality of trophic markers in S. thompsoni were likely related to their generally deeper residence depth in winter linked to a seasonal alternation of sexual and asexual generations.     

Bioenergetics of ephyra larvae of the scyphozoan jellyfish Aurelia aurita in relation to temperature and salinity

The body mass of Aurelia aurita ephyrae was better correlated with the diameter of the central disc than with the distance between opposite rhopaliae or distance between opposite lappet tips. Body dry weight (y, in μg) related to the disc diameter (x, in mm) through the equation y = 22.33 x ^1.99. The exponent 1.99 was significantly lower than that for the medusa stage, indicating a tendency to grow in diameter rather than in weight through the ephyra stage. The average ash-free dry weight (AFDW) of ephyrae was 38.0% of the dry weight. The AFDW/diameter relationship was used to convert measured diameters to body AFDW and calculate succession in body mass, daily ration, daily growth rate and gross growth efficiency. Effects of temperature (6, 9.5, 12, 15 and 18 °C) and salinity (17.5, 22, 26, 30.5 and 35 PSU) on these parameters and feeding were studied at saturated prey concentration (222 Artemia nauplii l⁻¹ initial concentration) by daily measurements over 10 d. There was a strong effect of temperature for total ingestion, growth rate, growth efficiency and final body mass of individual ephyrae, whereas the daily ration was not significantly different between the different temperatures. The experimental group kept at the highest temperature (18 °C) diverged the most, and ephyrae at this temperature ingested 2.7 times more and increased in weight 5.4 times more than at 6 °C. The average daily growth rate and gross growth efficiency of these ephyrae were 34.5% and 25.1%, respectively, significantly higher than at 6, 9.5 and 15 °C. Significant effects of salinity were shown for total ingestion, daily ration, daily growth rate and final weight, although only total ingestion and daily ration diverged sufficiently to show effects in a post-hoc test. This test showed that total ingestion was significantly different for all salinities except between 22 and 35 PSU and between 17.5 and 26 PSU. The daily ration for 35 PSU diverged from all other salinities, whereas none of the other salinities showed any significant differences. Thus, provided food in excess A. aurita can double its weight every 2 to 4 d, dependent on temperature and can therefore develop to the medusa stage in short time. Differences in environmental salinity in the range 17.5 to 35 PSU have little or no effect on growth rate and growth efficiency, whereas our results indicate that the full seawater salinity (35 PSU) causes significantly higher ingestion rate compared to lower salinities. Received: 11 January 1999 / Accepted: 11 May 1999     

Metabolic activity and elemental composition of krill and other zooplankton from Prydz Bay,Antarctica, during early summer (November–December)

Oxygen uptake and ammonia excretion rates, and body carbon and nitrogen contents were measured in krill (Euphausia superba) and eight other zooplankton species collected during November–December 1982 in the Prydz Bay, Antarctica. From these data, metabolic O:N ratios (by atoms), body C:N ratios (by weight) and daily metabolic losses of body carbon and nitrogen were calculated as a basis from which to evaluate seasonal differences in metabolism and nutritional condition. Comparison of the present data with mid-summer (January) data revealed that early-summer E. superba were characterized by higher metabolic O:N ratios (58.7 to 103, compared with 15.9 to 17.5 for mid-summer individuals). Higher O:N ratios of early-summer E. superba resulted largely from reduced ammonia excretion rates and, to a lesser degree, from slightly increased oxygen uptake rates. Body C:N ratios of E. superba were low in early-summer (3.8 to 4.2) compared with mid-summer krill (4.1 to 4.7) due to lowered body-carbon contents in the former (42.6 to 43.6% compared with 43.2 to 47.5% dry weight of midsummer individuals); gravid females formed an exception, since no seasonal differences in body elemental composition were detected for these. No significant changes in water content (75.3 to 81.4% wet wt) and nitrogen content (9.9 to 11.1% dry wt) in E. superba were evident between the two seasons. Seasonal differences in metabolic rates and elemental composition were less pronounced in a salp (Salpa thompsoni), but a higher metabolic O:N ratio occurred in early-summer individuals. Interspecific comparison of the seven remaining zooplankton species studied with twelve species from mid-summer zooplankton investigated in an earlier study indicated that higher metabolic O:N ratios in early-summer are characteristic not only of herbivore/filter-feeders, but also of some carnivores/omnivores. No relationship between metabolic O:N ratios and body C:N ratios was apparent either intraspecifically or interspecifically, within or between early-summer and mid-summer seasons.     

Dry weight, carbon and nitrogen components of caprellid amphipods (Crustacea) inhabiting the Sargassumyezoense community of Otsuchi Bay, northeastern Japan

Dry weight (DW), carbon (C) and nitrogen (N) contents in four species of caprellids, Caprella danilevskii, C. subinermis, C. penantis R-type, and C. verrucosa, inhabiting Sargassumyezoense (Yamada) in Otsuchi Bay, northeastern Japan, were measured based on live specimens. In all species males tend to grow larger than females in pereonite II length, which is used as an indicator of body length, and have greater dry weight, carbon and nitrogen contents. All three parameters (DW, C and N) had highly significant relationships against pereonite II length. Regressions of female pereonite II length against the three parameters significantly differed from males in the four species. Average percentages of carbon and nitrogen contents of the total dry weight ranged from 30.82 to 32.21% and 5.82 to 6.86%, respectively. The carbon/nitrogen ratio tended to decrease against increasing pereonite II length, e.g., 7.2 to 4.3 in C. danilevskii. Received: 25 November 1996 / Accepted: 31 July 1997     

Elemental and biochemical composition of salps (Tunicata: Thaliacea)

Carbon and nitrogen content have been measured in the solitary and aggregate generations of 11 species of salps. Regression equations for each species and generation permit estimation of carbon or nitrogen content as a function of length of live individuals. Different species of the same length may have nearly tenfold differences in carbon content. Fractionation and biochemical analysis of some samples revealed that the organic content of salps is approximately 80% protein. Ash-free dry weights average 27% of dry weights; mean carbon content is 29% of ash-free dry weight. Excess ash-free dry weight not accountable as organic material is thought to be water of hydration.     

Patterns of organic osmolytes in two marine bivalves, Macoma balthica, and Mytilus spp., along their European distribution

Patterns of nine intracellular free amino acids (FAA), which are utilized as organic osmolytes for salinity-induced cell volume regulation in marine osmoconformers, were compared in five Macoma balthica populations and seven Mytilus spp. populations along their European distribution. Three types of FAA patterns were classified within both taxa: a northern Baltic type, a southern Baltic type and an Atlantic/Mediterranean type which mainly differ regarding the share of alanine and taurine. Differences are discussed in relation to habitat salinity and population genetics. Along a salinity gradient, the total size of the intracellular FAA pool did not differ between sympatric M. balthica and Mytilus spp., and was significantly correlated with habitat osmolality in a range from 70 to 600 mmol kg⁻¹ H₂O (oligohaline to mesohaline) in both bivalves. In M. balthica, this correlation was mainly based on significant correlations of alanine (15–100 mmol kg⁻¹ DW), glycine (30–100 mmol kg⁻¹ DW) and taurine (0–70 mmol kg⁻¹ DW) with habitat osmolality. In Mytilus spp., only glycine (25–100 mmol kg⁻¹ DW) and taurine (4–180 mmol kg⁻¹ DW) were significantly correlated with habitat osmolality. The concentration of alanine was three times lower in Mytilus spp. than in M. balthica and did not correlate with habitat osmolality. Within a habitat osmolality range from 600 to 1,100 mmol kg⁻¹ H₂O (mesohaline to marine) the concentration of FAA remained constant in both taxa. It is suggested that under marine conditions additional organic osmolytes must become more important for cell volume regulation in Macoma and Mytilus.     

Temporal variation of Tubularia indivisa (Cnidaria, Tubulariidae) and associated epizoites on artificial habitat communities in the North Sea

We have analyzed the composition, diversity, density and biomass of a temporal series of samples taken in a Tubularia indivisa community, which dominates a shipwreck in the North Sea waters (N 51°23′,730–E 02°29′,790, 17 nautical miles from the coast, 30 m depth). This shipwreck has structures emerging up to 8 m above the seabed. Water temperature ranged from 4.2°C in March to 20.3°C in August. Salinity showed few variations around 33.9 psu. Bottom tidal currents followed a semi-diurnal cycle and were preferentially NE oriented with 84% of them in the range 0.25–0.75 m s⁻¹. The mean value for total suspended matter was 6.2 mg l⁻¹ with large variations on a monthly scale. The species richness of samples varied from 15 in October to 42 in August with a mean value of 33 species. Diversity indices were higher during autumn and winter because of the strong dominance of a few crustacean species during the warmer months. The total density of individuals ranged from 6,500 ind m⁻² in October to 445,800 ind m⁻² in July, most of these individuals belonging to the amphipod species Jassa herdmani. The biomass of the T. indivisa community varied from 9 g AFDW m⁻² in October to 1,106 g AFDW m⁻² in July, with T. indivisa itself constituting between 59 and 82% of the total biomass. The biomass of T. indivisa was positively correlated with species richness and with the density of 23% of the species identified on this community, suggesting that T. indivisa plays an important structural role in this habitat. This was further confirmed by the number of species associated with T. indivisa which was generally superior to 55% of the sorted species. Multivariate analysis indicated strong differences between spring/summer−autumn/winter assemblages mostly but not solely due to the abundance patterns of species. These findings support the conclusion that shipwrecks in Belgian waters allow the development of assemblages dominated by a high biomass of T. indivisa which in turn provides shelter for high densities and biomass of epizoites. These assemblages will further show large monthly variations in densities and composition due to large variation in T. indivisa biomass under an apparent repetitive annual cycle. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Brood size and chemical composition of Pareuchaeta norvegica (Crustacea: Copepoda) in Loch Etive,Scotland

Brood size and chemical composition (carbon, nitrogen, and ash) of Pareuchaeta norvegica in Loch Etive, Scotland have been determined. From 23 to 37 eggs were found in each egg mass (mean=30 eggs). The eggs are oval; the mean size of the longer axis is 438 m. Variation in egg mass (as dry weight) is related to increase in dry weight of the maternal body. an increase of 1 mg in maternal body dry weight produces an increase of 0.03 mg dry weight in the egg mass. Carbon content increases with increasing body dry weight both in pre-spawning and spent females, but nitrogen content decreases with increasing body weight. Thus, the carbon:nitrogen ratio increases with increasing body dry weight. The carbon content of an egg mass ranges from 61.4 to 65.3%, and nitrogen content from 9.6 to 13.2% of the total dry weight. Some examples of partitioning of chemical components of the maternal body into the egg mass are also described.     

Elemental sulfur in the gills of three species of clams containing chemoautotrophic symbiotic bacteria: a possible inorganic energy storage compound

Sulfur content and fine structure were studied for tissues of three species of clams, Lucinoma annulata, Calyptogena elongata and Lucina floridana, which inhabit sulfide-rich environments and whose gills harbor symbiotic sulfur bacteria. Lucinoma annulata and C. elongata were dredged from the Santa Barbara basin, California, USA, at a depth of 480 to 490 m, and Lucina floridana were dug from below the roots of seagrasses in Saint Joseph Bay, Florida, at a depth of 0.25 to 2m. Foot tissue of Lucinoma annulata, without symbionts, had a total sulfur content of 1.4±0.1 (SD) mg 100 mg^-1 dry weight of tissue (%DW). The symbiont-containing gill tissue of different individuals of L. annulata varied in color from dark red to pale yellow, and the total sulfur content was 2.5±0.4% DW in red gills and was 5.6±3.3 % DW in the yellowest gills. Maintenance of L. annulata in the laboratory for 21 d in the absence of sulfide resulted in the loss from the gill of yellow deposits which were elemental sulfur in the form of liquid-crystalline sulfur globules rather than solid orthorhombic sulfur crystals. The foot tissue did not contain elemental sulfur. When examined by freeze-etch microscopy, sulfur globules were found only within bacteria and not in the animal host cytoplasm. Sulfur globules were confined to the periplasmic space of the bacteria. C. elongata and Lucina floridana resembled Lucinoma annulata in the physical form and distribution of elemental sulfur. The absence of elemental sulfur in the animal cytoplasm suggests that its formation from sulfide is not a detoxification scheme to protect animal tissue from sulfide toxicity. The sulfur deposits probably represent inorganic energy reserves that permit the symbiotic bacteria to function even during the temporary absence of external sulfide.     

Changes in metabolic substrates during early development in anchoveta Engraulis ringens (Jenyns 1842) in the Humboldt Current

We assessed the ontogenetic changes in protein content and free amino acids (FAA) in eggs and early larvae of Engraulis ringens (anchoveta) off central Chile on different dates during the spawning season. On all sampling dates, a reduction in embryonic yolk-sac volume, proteins and FAA concentrations occurred during development. Protein electrophoresis (SDS–PAGE) of eggs and larvae showed at least 22 protein bands: 11 were consumed early and not detected after hatching. The proportion of essential FAA (EFAA) was higher than the proportion of non-essential FAA (NEFAA) in early eggs and in 7 day-old larvae (82.5-73% EFAA respectively). During egg development, the FAA pool was dominated by leucine, alanine and lysine, three amino acids contributing 35–44% of the total FAA in eggs. During larval development, histidine was the most abundant FAA. In July, total FAA constituted 13–18% of the egg dry weight. A similar proportion (45–51%) occurred in July between protein plus FAA and total lipids. The differences in egg size during the spawning season along with variability in batch composition suggests that the female spawning condition is a major factor determining egg quality and early offspring success.     

Organic and calorific content of the body tissues of deep-sea elasipodid holothurians in the northeast Atlantic Ocean

Data are presented on the biochemical analysis of protein, lipid, carbohydrate and ash for the ovary, testis, gut and body wall of six species of elasipodid holothurians at a variety of stations in the Porcupine Seabight and Abyssal Plain (northeast Atlantic Ocean). Samples were collected between 1980 and 1982. The species investigated were Benthogone rosea, Laetmogone violacea, Oneirophanta mutabilis, Deima validum, Benthodytes sordida and Psychropotes longicauda. Lipid was the dominant soluble constituent of the ovary and protein was dominant in the testis. Tissue calorific content was determined from the biochemical data and by microbomb calorimetry. The ovaries had higher calorific values than the other body tissues when determined by both methods. Comparison of the two methods suggests that the calorific values for testis, gut and body wall are significantly different, and this may be a function of the calcium-carbonate content of these tissues. Total-body calorific value varied from 24.26 J mg^-1 ash-free dry weight (AFDW) (5 799 cal g^-1 AFDW) to 25.43 J mg^-1 AFDW (6 078 cal g^-1 AFDW). Total holothurian-body calorific biomass at depths of 1 010, 2 000 and 4 000 m is 0.49, 3.69, and 0.25 kJ m^-2 (118, 882, 59.4 cal m^-2), respectively. Holothurians form a significant store of energy in the deep sea.     

Vertical heterogeneity in physiological characteristics of Ulva spp. mats

In eutrophic areas, green macroalgae are frequently and for long periods arranged in mats, resulting in a steep light gradient. This study investigates the effect of this gradient on physiological characteristics [tissue nitrogen content, maximal photosynthetic efficiency (F_v/F_m), glutathione levels and redox ratio, absorbance and absorption spectra] of the green macroalga Ulva spp. Mats were sampled during the build-up (June), stationary (July), and decomposing (September) phases of a macroalgal bloom in the Veerse Meer, a eutrophic brackish (salinity 15–20 psu) lake in the southwest Netherlands. Water samples were taken for nutrient analyses. At all three sampling dates, the mats were composed almost entirely of Ulva spp.; in September the mats were in decay and covered with silt and epiphytes. In June and July, total dissolved inorganic nitrogen concentration (DIN) of the water within the mat was significantly higher than outside the mat. Pronounced vertical differences were found in tissue N, F_v/F_m values, total glutathione levels, glutathione redox ratios, and absorbance. In June and July, tissue N decreased from over 2.2% dry weight (DW; N-sufficient) in the bottom layers to around 1% DW (minimum level for survival) in the top layers. Wide-band absorption increased with depth in the mat and throughout the season, probably due to higher Chl a and b and lutein contents. The shape of the absorption spectrum was similar for all layers. The absorption of the silt/epiphyte film on the top Ulva layer was highest; its absorption spectrum (high absorption in the 500–560 nm range) indicates that the film on the top layers of the macroalgal mats mainly consisted of diatoms. In June, F_v/F_m and the glutathione redox ratio of the algae increased with depth in the layer, while total glutathione decreased. Low F_v/F_m values in the bottom and middle layers in September reflect the bad condition of the algae; the mats were largely decaying. It is concluded that multiple growth-limiting gradients occur in macroalgal mats: upper layers suffer from nitrogen limitation and photoinhibition while bottom layers are light limited. The algae in the mat acclimatize to low light conditions by increasing their absorption through increased pigment contents and by higher photosynthetic efficiency during the build-up and stationary period. This study qualifies the glutathione redox ratio as a promising candidate for stress indicator in macroalgae and provides suggestions for its further development.Communicated by S.A. Poulet, Roscoff     

Contribution of salps to carbon flux of marginal ice zone of the Lazarev Sea, southern ocean

In order to estimate the in situ grazing rates of Salpa thompsoni and their implications for the development of phytoplankton blooms and for the sequestration of biogenic carbon in the high Antarctic, a repeat-grid survey and drogue study were carried out in the Lazarev Sea during austral summer of 1994/1995 (December/January). Exceptionally high grazing rates were measured for S. thompsoni at the onset of a phytoplankton bloom (0.2 to 0.8 μg chlorophyll a l⁻¹) in December 1994, with up to ≃160 μg of plant pigments consumed by an individual salp of 7 to 10 cm length per day. Dense salp swarms extended throughout the marginal ice zone, consuming up to 108% of daily phytoplankton production and 21% of the total chlorophyll a stock. Due to the much faster sinking rates and higher carbon content of salp faecal pellets, the efficiency of downward carbon flux through salps is much higher than through the other major grazers, krill and copepods. S. thompsoni can thus export large amounts of biogenic carbon from the euphotic zone to the deep ocean. With the observed ingestion rates during December 1994, this flux could have attained levels of up to 88 mg C m⁻² d⁻¹, accounting for the bulk of the vertical transport of carbon in the Lazarev Sea. However, in January 1995, when phytoplankton concentrations exceeded a threshold level of 1.0 to 1.5 μg chlorophyll a l⁻¹, salps experienced a drastic reduction in their feeding efficiency, possibly as a result of clogging of their filtering apparatus. This triggered a dramatic reversal in the relationship, during which a dense phytoplankton bloom developed in conjunction with the collapse of the salp population. Increases in the biomass and geographic range of the tunicate S. thompsoni have occurred in several areas of the southern ocean, often in parallel with a rise in sea-surface temperature during sub-decadal periods of warming anomalies. Received: 10 August 1997 / Accepted: 21 October 1997     

An assessment of seaweed decomposition within a southern Strait of Georgia seaweed community

An assessment of litter and detritus decomposition and nitrogen content of decomposing litter is presented for ten important seaweeds within a southern Strait of Georgia (British Columbia, Canada) seaweed community sampled from August 1975 until October 1976. Litter decomposition rates varied among species with the time required for litter to disappear from litter bags ranging from 6 d for the lamina of Nereocystis luetkeana to about 70 d for Fucus distichus. Decomposition was characterized by an accelerating increase in the nitrogen: dry weight ratio of remnant litter as decomposition proceeded. Iridaea cordata detritus decomposed most rapidly, at 5.7% d^-1, while rates for Gigartina papillata, N. luetkeana, Laminaria saccharina and Laminaria groenlandica ranged from 1.8 to 3.6% d^-1. The remaining species decomposed more slowly. There was a tendency toward more rapid decomposition with decreasing crude fibre content and detritus particle size; however, it appears that morphology, habitat and growth rate are also correlated with relative decomposition rates. Of 43 taxa identified within quantitative litter collections, F. distichus (41%), I. cordata (26%), N. luetkeana (27%) and Laminaria spp. (4%) accounted for 98% of total deposition with mean peak accumulation occurring in August and September from a low near zero in January and February. Litter distribution was patchy, with most litter decomposing near its place of deposition. The application of litter decomposition rates to measured litter accumulation in a mathematical simulation of decomposition predicted the rate of seaweed litter decomposition to peak at about 1.1 g AFDW (ash-free dry weight) m^-2 d^-1 in early September from a mid-winter low near zero. In total, 56±4% of decomposing litter formed detritus, with the remainder being released as soluble matter. The annual contribution of seaweed litter biomass to detrital pathways from our study site was calculated to be 152 g AFDW m^-2.