Phosphorus- and nitrogen-limited photosynthesis and growth of Gracilaria tikvahiae (Rhodophyceae) in the Florida Keys: an experimental field study

The relative effects of NH ₄ ⁺ (N) and PO ₄ ^3- (P) on growth rate, photosynthetic capacity (P_max), and levels of chemical constituents of the red macroalga Gracilaria tikvahiae McLachlan were assayed during winter and summer, 1983 in inshore waters of the Florida Keys by using in-situ cage cultures. During winter, both N and P enrichment enhanced growth over that of ambient seawater; however, P rather than N accounted for more (60%) of the increased winter growth. During summer, P, but not N, enhanced growth over ambient seawater and accounted for 80% of increased growth. Similarly, P_max was enhanced by both P and N during winter (but mostly by P) and only by P during summer. Elevated C:P, C:N and N:P ratios of G. tikvahiae tissue during winter, but only C:P and N:P ratios during summer, support the pattern of winter N and P limitation and summer P-limitation. This seasonal pattern of N vs P limited growth of G. tikvahiae appears to be a response to seasonally variable dissolved inorganic N (twofold greater concentrations of NH ₄ ⁺ and NO ₃ ^- during summer compared to winter) and constantly low to undetectable concentrations of PO ₄ ^3- . Mean C:P and N:P ratios of G. tikvahiae tissue during the study were 1 818 and 124, respectively, values among the highest reported for macroalgae.     

Spatial and temporal variation in the elemental and stable isotopic content of the seagrasses <Emphasis Type="Italic">Posidonia oceanica</Emphasis> and <Emphasis Type="Italic">Cymodocea nodosa</Emphasis> from the Illes Balears,Spain

Morphology, elemental content and isotopic composition of leaves of the seagrasses Posidonia oceanica and Cymodocea nodosa were highly variable across the Illes Balears, a Spanish archipelago in the western Mediterranean, and varied seasonally at one site in the study area. The data presented in this paper generally expand the reported ranges of nitrogen, phosphorus, iron and arsenic content and δ¹³C and δ¹⁵N for these species. Nitrogen and phosphorus content of P. oceanica leaves also showed significant seasonal variability; on an annual basis, P. oceanica leaves averaged 1.55% N and 0.14% P at this monitoring site. Both N and P were more concentrated in the leaves in winter than in summer, with winter maxima of 1.76% N and 0.17% P and summer minima of 1.34% N and 0.11% P. There was no significant annual pattern observed in the δ¹³C of P. oceanica leaves, but there was a repeated 0.6‰ seasonal fluctuation in δ¹⁵N. Mean annual δ¹⁵N was 4.0‰; δ¹⁵N was lowest in May and it increased through the summer and autumn to a maximum in November. Over the geographic range of our study area, there were interspecific differences in the carbon, nitrogen and phosphorus content of the two species. Posidonia oceanica N:P ratios were distributed around the critical value of 30:1 while the ratios for C. nodosa were lower than this value, suggesting P. oceanica we collected was not consistently limited by N or P while C. nodosa tended toward nitrogen limitation. Nutrient content was significantly correlated to morphological indicators of plant vigor. Fe content of P. oceanica leaves varied by a factor of 5×, with a minimum of 31.1 μg g⁻¹ and a maximum of 167.7 μg g⁻¹. Arsenic was present in much lower tissue concentrations than Fe, but the As concentrations were more variable; the maximum concentration of 1.60 μg g⁻¹ was eight times as high as the minimum of 0.20 μg g⁻¹. There were interspecific differences in δ¹³C of the two species; C. nodosa was consistently more enriched (δ¹³C = −7.8 ± 1.7‰) than P. oceanica (−13.2 ± 1.2‰). The δ¹³C of both species decreased significantly with increasing water depth. Depth related and regional variability in the δ¹³C and δ¹⁵N of both species were marked, suggesting that caution needs to be exercised when applying stable isotopes in food web analyses.     

Growth patterns of Codium fragile ssp. tomentosoides in response to temperature,irradiance, salinity,and nitrogen source

Seasonal patterns of growth, reproduction, and productivity of Codium fragile spp. tomentosoides (van Goor) Silva were monitored at 3 locations in Rhode Island. Maximal growth occurred during the summer and was more significantly correlated with temperature than any other factor measured in this study. Multiple correlation models suggested an interaction between temperature, irradiance, and available nitrogen. Maximal reproduction occurred in late summer and early fall. The maximal productivity, based on harvested quadrats, was 2. 10 g dry weight m^-2 day^-1. A large amount (up to 87.3%) of the annual production entered the detrital food chain during the winter by fragmentation of the thallus. Culture studies examined the effects of temperature (6° to 30°C), irradiance (7 to 140 E m^-2 sec^-1), daylength (8 h light: 16 h dark to 24 h light: O h dark) and salinity (6 to 48) on growth. Differentiated thalli grew over a broad range of experimental conditions, with maximal growth at 24°C, 24 to 30 S, a minimal irradiance of 28 E m^-2 sec^-1, and 16 h daylength. The effect of increasing daylength was due to increased total daily irradiance rather than to a true photoperiodic effect. Undifferentiated sporelings survived and grew in a narrower range of environmental conditions than thalli. c. fragile spp. tomentosoides grew equally well with nitrate, nitrite, ammonium, and urea as a nitrogen source. The addition of NaHCO₃ stimulated growth at levels of 2.4 to 4.8 mM, suggesting an inorganic carbon limitation in static cultures. This study supports the hypothesis that the in situ seasonal growth pattern of c. fragile spp. tomentosoides is primarily due to the interaction of temperature and irradiance.     

Seasonal growth and composition of fronds of Macrocystis pyrifera in the Falkland Islands

Frond growth of Macrocystis pyrifera in the Falkland Islands was monitored in shallow coastal water from December 1985 to March 1987, and at a different site in deeper water from December 1985 to June 1986. Growth rates in the deeper bed were generally higher than those recorded in the coastal zone. At both sites, node initiation and elongation rate fluctuated according to the seasonal pattern of light or water temperature. In the shallow coastal area, nitrate was abundant in the winter and below detection levels during late spring and summer. Correlation analysis suggests that the production of the fronds of the giant kelp in this area was probably inhibited during the summer months by extremely low concentrations of nutrients. Internal nitrogen was exhausted approximately one month after a sharp decline in ambient nitrate concentration, and carbon reserves were formed. In the deeper bed of M. pyrifera, nitrogen was abundant all year round and the production of the fronds reflected the seasonal pattern of light or water temperature. The nitrogen content of the tissue probably did not drop below a level that limited production, and no internal carbon reserves were accumulated.     

Seasonal variation in total and soluble tissue nitrogen of Pleurophycus gardneri (Phaeophyceae: Laminariales) in relation to environmental nitrate

Seasonal variations in tissue nitrogen (ethanol soluble nitrate and ninhydrin positive substances, as well as total nitrogen) of different thallus parts of Pleurophycus gardneri Setchell and Saunders were monitored simultaneously with ambient seawater nitrate from 1982 until 1984 in Bamfield, Vancouver Island, British Columbia, Canada. A trend of low, nearly zero levels in ambient nitrate typical for the area in late spring and early summer normally contrasts with average nitrate concentrations of 10 mol NO₃ ^- l^-1 in late fall and winter. Total nitrogen content was greater in the perennial thallus parts, stipe and holdfast than in the annual blade and peaked in fall and early winter. The longitudinal thallus distribution of nitrate revealed a distinct and significant concentration of nitrate in the haptera reaching at maximum 8% nitrate-N of the internal total nitrogen. Internal nitrate concentration ranged from 20 to 5 000 times the ambient nitrate concentration in the midrib, and from 40 to 3 100 times in the wing, while the range was greatest with 400 to 14 000 times in the haptera. P. gardneri contained at most about 7 mol NO₃ ^- g fresh wt^-1 in the blade, which corresponds to about 6% of total tissue nitrogen. Ninhydrin positive substances comprised the major portion of the soluble N pool in P. gardneri and showed a pronounced seasonality. Concentrations of ninhydrin positive substances ranged from 20 to 800 g N g fresh wt^-1 in the midrib and in the wing. In the stipe, ninhydrin positive substances varied from 180 to 2 200 g N g fresh wt^-1, and from 250 to 1 200 g N g fresh wt^-1 in the haptera. Evidence is given that (1) the perennial parts, stipe and haptera of P. gardneri contain the majority of nitrogen products independent of season and ontogenetic stage; (2) ninhydrin positive substances are the most abundant internal nitrogen constituents; (3) the low N values in the blade in summer suggest a nitrogen limited growth; and (4) nitrate may not be the predominant external nitrogen source.     

Persistent blooms of surf diatoms along the Pacific coast,USA

Productivity was studied in two diatom species, Chaetoceros armatum T. West and Asterionella socialis Lewin and Norris, which form persistent dense blooms in the surf zone along the Pacific coast of Washington and Oregon, USA. Past observations have shown that surf-diatom standing stock usually declines in summer along with concentrations of nitrate and ammonium. Using the ¹⁴C method, photosynthetic rates in natural surf samples were measured monthly for one year (October 1981 through September 1982) at a study site on the Washington coast. Also measured were temperature, salinity, dissolved nutrients, particulate carbon and nitrogen (used as estimates of phytoplankton C and N), and chlorophyll a. Assimilation numbers (P _max) were higher in summer (5 to 8 g C g^-1 chl a h^-1) than in winter (3 to 4gC). Specific carbon incorporation rates (µ_max) showed no obvious seasonality, mostly falling within the range of 0.09 to 0.13 g C g^-1 C(POC) h^-1. The discrepancy between the seasonal trends for chlorophyll-specific and carbon-specific rates reflects a change in the carbon-to-chlorophyll ratio. Because of seasonal differences in daylength and light intensity, daily specific growth rates () are thought to be higher in summer than in winter. Neither ammonium enrichment assays nor particulate carbon-to-nitrogen ratios provided convincing evidence for nitrogen limitation during summer, and the observed changes in diatom abundance cannot be explained on this basis. Both the high diatom concentrations and their seasonal variations probably are due mainly to factors other than growth rates; two factors considered important are diatom flotation and seasonal changes in wind-driven water transport. C. armatum usually dominates the phytoplankton biomass in the surf zone, and evidence suggests that this species is strongly dominant in terms of primary production.Contribution No. 1391 of the School of Oceanography, University of Washington, Seattle, Washington, USA     

Marine diatoms grown in chemostats under silicate or ammonium limitation. III. Cellular chemical composition and morphology of Chaetoceros debilis,Skeletonema costatum,and Thalassiosira gravida

Three marine diatoms, Skeletonema costatum, Chaetoceros debilis, and Thalassiosira gravida were grown under no limitation and ammonium or silicate limitation or starvation. Changes in cell morphology were documented with photomicrographs of ammonium and silicate-limited and non-limited cells, and correlated with observed changes in chemical composition. Cultures grown under silicate starvation or limitation showed an increase in particulate carbon, nitrogen and phosporus and chlorophyll a per unit cell volume compared to non-limited cells; particulate silica per cell volume decreased. Si-starved cells were different from Si-limited cells in that the former contained more particulate carbon and silica per cell volume. The most sensitive indicator of silicate limitation or starvation was the ratio C:Si, being 3 to 5 times higher than the values for non-limited cells. The ratios Si:chlorophyll a and S:P were lower and N:Si was higher than non-limited cells by a factor of 2 to 3. The other ratios, C:N, C:P, C:chlorophyll a, N:chlorophyll a, P:chlorophyll a and N:P were considered not to be sensitive indicators of silicate limitation or starvation. Chlorophyll a, and particulate nitrogen per unit cell volume decreased under ammonium limitation and starvation. NH₄-starved cells contained more chlorophyll a, carbon, nitrogen, silica, and phosphorus per cell volume than NH₄-limited cells. N:Si was the most sensitive ratio to ammonium limitation or starvation, being 2 to 3 times lower than non-limited cells. Si:chlorophyll a, P:chlorophyll a and N:P were less sensitive, while the ratios C:N, C:chlorophyll a, N:chlorophyll a, C:Si, C:P and Si:P were the least sensitive. Limited cells had less of the limiting nutrient per unit cell volume than starved cells and more of the non-limiting nutrients (i.e., silica and phosphorus for NH₄-limited cells). This suggests that nutrient-limited cells rather than nutrient-starved cells should be used along with non-limited cells to measure the full range of potential change in cellular chemical composition for one species under nutrient limitation.Contribution No. 943 from the Department of Oceanography, University of Washington, Seattle, Washington 98195, USA.     

Seasonal variations in distribution patterns of sympagic meiofauna in Arctic pack ice

During two expeditions of the R.V. Polarstern to the Arctic Ocean, pack ice and under-ice water samples were collected during two different seasons: late summer (September 2002) and late winter (March/April 2003). Physical and biological properties of the ice were investigated to explain seasonal differences in species composition, abundance and distribution patterns of sympagic meiofauna (in this case: heterotrophs >20 µm). In winter, the ice near the surface was characterized by extreme physical conditions (minimum ice temperature: –22°C, maximum brine salinity: 223, brine volume: 5%) and more moderate conditions in summer (minimum ice temperature: –5.6°C, maximum brine salinity: 94, most brine volumes: 5%). Conditions in the lowermost part of the ice did not differ to a high degree between summer and winter. Chlorophyll a concentrations (chl a) showed significant differences between summer and winter: during winter, concentrations were mostly <1.0 µg chl a l^–1, while chl a concentrations of up to 67.4 µmol l^–1 were measured during summer. The median of depth-integrated chl a concentration in summer was significantly higher than in winter. Integrated abundances of sympagic meiofauna were within the same range for both seasons and varied between 0.6 and 34.1×10³ organisms m^–2 in summer and between 3.7 and 24.8×10³ organisms m^–2 in winter. With regard to species composition, a comparison between the two seasons showed distinct differences: while copepods (42.7%) and rotifers (33.4%) were the most abundant sea-ice meiofaunal taxa during summer, copepod nauplii dominated the community, comprising 92.9% of the fauna, in winter. Low species abundances were found in the under-ice water, indicating that overwintering of the other sympagic organisms did not take place there, either. Therefore, their survival strategy over the polar winter remains unclear.Communicated by O. Kinne, Oldendorf/Luhe     

Seasonal growth in Laminaria longicruris: Relations with dissolved inorganic nutrients and internal reserves of nitrogen

Observations have been made on seasonal fluctuations in dissolved inorganic nutrients, internal reserves of nitrogen and growth rates in Laminaria longicruris. The onset of winter growth in shallow-water stations (6 and 9 m) correlated well with improved dissolved nitrate conditions in the sea. During the winter, reserves of NO ₃ ^- were accumulated by the plants and reached maximum values of 150 moles per g fresh weight in March. This represents a concentration factor of approximately 28,000 over the ambient levels, or an internal nitrogen reserve of 2.1% of the dry weight of the tissue. Depletion of this nitrogen pool followed the disappearance of the external NO ₃ ^- with a lag period of up to 2 months. Rapid kelp growth was measured during this period. Reserves of organic nitrogen also reached maximum values in March and declined slowly throughout the summer into autumn. It is suggested that the combined inorganic and organic nitrogen reserves sustain the rapid growth rates into July and at reduced rate through the late summer. Fertilization of an experimental perimental kelp bed with NaNO₃ increased the internal plant reserves of NO ₃ ^- and produced a much improved summer growth rate. The enriched plants developed very small reserves of carbohydrate during the rapid summer growth phase.NRCC No. 15549.     

Seasonal patterns of growth and storage in Laminaria longicruris in relation to differing patterns of availability of nitrogen in the water

The annual cycles of growth and of internal storage of nitrogen and carbon in Laminaria longicruris were compared at three sites in Nova Scotia, Canada. One site had abundant nitrogen all year round, on account of local upwelling, one had abundant nitrogen for only four months and the last was intermediate in this respect. Where nitrogen was abundant year round, growth followed the seasonal pattern of light, and there was little storage of either carbon or inorganic nitrogen. Where nitrogen was only abundant in the sea in winter, growth accelerated at this time and continued throught early summer while internal reserves of nitrogen were utilized. Carbon was stored in summer and used for growth in the fall. At the intermediate site, larger reserves of nitrogen were built up in winter and a high growth rate was maintained through most of the summer. Smaller carbon reserves were accumulated and growth was minimal in winter. Experiments in which plants were transplanted between sites were not conclusive, but suggested that plants at different sites may have genetic differences.     

Seasonal effects of light,temperature, nutrient concentration and salinity on the physiology and growth of Caulerpa paspaloides (Chlorophyceae)

Caulerpa paspaloides (Bory) Greville were collected during the winter and summer (1978 to 1979) from the Florida Keys, USA. Thalli collected during the winter photosynthesized more efficiently at low light intensities (I_c<1, I_k=38 Exm^-2xs^-1) than did thalli collected in the summer (I_c=13, I_k=111 Exm^-2xs^-1). Summer thalli exhibited higher P_max values (2.20 mgO₂xg^-1 dry wtxh^-1) than winter thalli (1.70 mg O₂xg^-1 dry wtxh^-1). Rates of rhizome elongation and frond initiation were strongly inhibited by winter temperatures. The maximum lethal temperature for summer thalli was 37° to 38°C as measured by both growth and photosynthesis. No evidence of nitrogen or phosphorus limitation was found. Relatively minor reductions in salinity (3S) resulted in significant increases in rhizome apex motality. Results indicate that low winter temperatures are responsible for reduced winter growth rates previously reported for the Key Largo population. Increased photosynthetic efficiency at low light intensities and preferential maintenance of rhizome elongation over frond initiation appear to allow this tropical macroalga to optimize growth and survival during the winter.     

Anti-grazing activity and seasonal variation of dimethylsulfoniopropionate-associated compounds in the invasive alga <Emphasis Type="Italic">Codium fragile</Emphasis> ssp. <Emphasis Type="Italic">tomentosoides</Emphasis>

In this study, we present evidence that the invasive alga Codium fragile ssp. tomentosoides is chemically defended against grazing by a wound-activated chemical defense involving dimethylsulfoniopropionate (DMSP) and the products of its cleavage, dimethylsulfide (DMS), and acrylic acid (AA). DMSP in C. fragile ssp. tomentosoides was present throughout the year, but concentrations varied seasonally and were highest in the winter. Intra-thallus variation was neither large, nor consistent over time. High DMSP concentrations were uncommon among northwestern Atlantic macrophytes. Of 26 other species tested, only two, Ulva lactuca and Polysiphonia harveyi contained concentrations comparable to, or higher than, those of C. fragile ssp. tomentosoides. DMS and AA, both individually and together, deterred grazing by the green sea urchin Strongylocentrotus droebachiensis at “natural” concentrations. These results suggest that DMS and AA contribute to the avoidance of C. fragile ssp. tomentosoides by S. droebachiensis. As a result, the production of DMSP and its subsequent cleavage, upon injury, may reduce herbivory on C. fragile ssp. tomentosoides and contribute to its success.     

Stoichiometry of mesozooplankton in N- and P-limited areas of the Baltic Sea

The Baltic Sea is a very suitable site for stoichiometric studies, since its subbasins differ in their concentration of elemental components, and primary production can therefore be either nitrogen or phosphorus limited. To reveal if the nutrient limitation of mesozooplankton mirrors that of the primary producers, carbon, nitrogen and phosphorus content of both seston and grazers (Acartia sp., Centropages hamatus, Daphnia cristata, Eurytemora affinis, Limnocalanus macrurus, Temora longicornis) were measured in midsummer in the Baltic proper, the Gulf of Finland and the Gulf of Bothnia. The mineral ratios of the different taxa were equal, apart from L. macrurus with notably higher C:P and N:P ratios. Molar C:N ratios were relatively stable (5.1-6.3), whereas C:P and N:P ratios fluctuated more (41-144 and 6.6-24). However, zooplankton elemental composition and limitation did not depend on the limiting nutrient of the phytoplankton, the seston mineral ratio or the sea area. Both the seston-zooplankton elemental imbalance and the food threshold ratio indicated phosphorus limitation of most of the grazers. While L. macrurus may be C or N limited, the possible P deficiency of the other studied taxa suggests that the Baltic Sea zooplankton may act as a potential phosphorus sink, as the freshwater secondary producers do.     

Spatio-temporal variations in the diet and stable isotope composition of the Argentine hake <Emphasis Type="Italic">Merluccius hubbsi</Emphasis> Marini, 1933 of the continental shelf of southeastern Brazil

The feeding ecology of Merluccius hubbsi was investigated in 2 regions of SE Brazil. The major food sources for the hakes were fish, crustaceans, and squid. In the upwelling system of Cabo Frio, the diet was very similar in the summers of 2001/2002 and spring 2002; fish were the most important prey followed by crustaceans. In Ubatuba, euphausiids were an important prey during the winter 2001 (100 m), while in the summer 2002, fish and amphipods predominated in the diet in the shallower site (40 m) and squid in the deeper site (100 m). The hakes showed temporal differences in stable isotope signatures in both regions, while C:N ratios varied only in Cabo Frio. δ¹⁵N and δ¹³C (bulk and corrected for lipid content) increased with fish length, which seems to be related to the increasing importance of fish and decreasing importance of euphausiids and amphipods in the diet of larger hakes. The mean trophic level of 3.7 for M. hubbsi was estimated using δ¹⁵N of bivalves as baseline and the fractionation of 3.4‰ between trophic levels.     

Comparative feeding preference of Strongylocentrotus droebachiensis (Echinoidea) for the invasive seaweed Codium fragile ssp. tomentosoides (Chlorophyceae) and four other seaweeds

Laboratory experiments conducted during 1987 on Appledore Island, Maine, USA, tested whether feeding preference or the absence of an attractant was the cause for the occurrence of beds of Codium fragile ssp. tomentosoides (herein referred to as Codium fragile) within rocky barrens grazed clear of kelp by the sea urchin Strongylocentrotus droebachiensis. Consumption of C. fragile in single-diet experiments (1 seaweed/sea urchin) was highly variable and was not significantly different from that for several other seaweeds (Agarum cribrosum, Ascophyllum nodosum, Chondrus crispus, and Laminana saccharina) important in the field diet of the green sea urchin. In multiple-diet experiments (5 seaweeds/sea urchin) significantly less Codium fragile was eaten than Chondrus crispus, but significantly more Codium fragile was eaten than A. cribrosum. Chemosensory experiments suggest that C. fragile does not attract the sea urchin. Sea urchins are unable to detect C. fragile but will eat it when they come in contact with it.     

Composition biochimique des copépodes de l'hyponeuston de Méditerranée Nord occidentale. Poids sec et analyse élémentaire du carbone,de l'hydrogène et de l'azote

Dry weight, total carbon, hydrogen and nitrogen contents were studied in Anomalocera patersoni, Pontella mediterranea, P. lo biancoï and Labidocera wollastoni as a function of sex and developmental stage. 629 individuals were analysed over the year. Protein content was calculated from the nitrogen values, and results are presented as percent dry weight. C:H and C:N ratios were also determined. The lowest contents of carbon (32.4%) and nitrogen (9.3%) were determined for female L. wollastoni, the highest carbon content (43.3%) for female P. lo biancoï, and the highest nitrogen content (11.5%) for female P. lo biancoï and male A. patersoni. This range agrees with the data in the literature for marine copepods. According to available data on biochemical composition of zooplankton in relation to depth, the Pontellidae contain a high amount of proteins. The carbon:nitrogen ratio displays great stability within a species, indicating a constant elementary composition during development from copepodite to adult. Nevertheless, there is a statistically significant discrepancy between the C:N ratios for A. patersoni and P. mediterranea which is due to a higher rate of increase in carbon content in A. patersoni. As a whole, interspecific variations were small (C:N ranged between 3.4 and 3.8) compared to those recorded in true planktonic species. This appears to be an important characteristic of the Pontellidae, in contrast with other, more widely distributed copepods, and probably is related to the peculiarities of their biotope, the ultrasuperficial layer.     

Population dynamics and reproductive biology of the commensal isopod Colidotea rostrata (Crustacea: Isopoda: Idoteidae)

Colidotea rostrata (Benedict, 1898) is the only known commensal idoteid isopod, living on and mimicking the color of two northeastern Pacific sea urchins of the genus Strongylocentrotus. The population dynamics and reproductive biology of C. rostrata on its host urchins were studied at a low rocky intertidal area in southern California (33°40N; 118°30W) from December 1984 to December 1986. Isopod populations remained relatively stable throughout the 2 yr study, with isopods inhabiting an average of 56.1% of the urchins at 6.1±0.6 (x±2 SE) isopods per urchin. Female isopods reach sexual maturity at a length of 6.8 mm. Fecundity in C. rostrata averaged 11.8±0.9 (x±2 SE) embryos per brood, and increased with female body size. Breeding occurs all year long in C. rostrata, with a main reproductive period between the warmer spring and summer months. Newly released mancas and juvenile isopods were present during all months of the study. C. rostrata differs from the free-living Idoteidae in its smaller maximum size, reduced fecundity, 1:1 sex ratio, and low juvenile mortality. These features may represent adaptations to a commensal life style that reflect a reduced mortality pressure on these isopods.     

Nitrogen-fixation by cyanobacteria associated withCodium fragile (Chlorophyta): Environmental effects and transfer of fixed nitrogen

N₂-fixation associated with the green macroalgaCodium fragile subsp.tomentosoides (van Goor) Silva from Long Island, New York, USA, was attributable to several species of endophytic cyanobacteria. Rates of N₂-fixation ranged from 0.03 to 3.2µg N g^–1 dry wt h^–1 in freshly collected plants from several sites. Growth of the cyanobacteria appeared to be light-limited, due to the transmission of only 5 to 10% of incident light through the pigmented surface-layer of the macroalga. Daily irradiance was the most important factor determining both abundance of cyanobacterial cells and rate of N₂-fixation. The rate was also affected by instantaneous irradiance, and increased twofold from dark to ambient surface irradiance. Rates were reduced at low temperature (8°C) but showed no temperature effect between 12° and 26°C. External concentrations of dissolved inorganic nitrogen (DIN) up to 20µM did not influence N₂-fixation rate, but long-term exposure to 60µmol l^–1 d^–1 of NH ₄ ⁺ caused a reduction in the rate. InC. fragile grown under high daily irradiance and low external DIN concentration, ~50% of the assimilated-N was attributable to N₂-fixation. However, chlorophyllb extracted from plants grown with¹⁵N₂ showed an atom % excess¹⁵N of less than 0.1, suggesting that only a small proportion of the bacterially fixed-N was transferred to the seaweed. The association betweenC. fragile and its endophytic cyanobacteria appears to be based primarily on microhabitat suitability, rather than mutual metabolic dependence. It is doubtful that N₂-fixation by cyanobacteria is important to the ecological success of this seaweed species.     

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.     

Introduced versus native subspecies of Codium fragile: how distinctive is the invasive subspecies tomentosoides?

After its introduction, the green alga Codium fragile (Sur.) Hariot ssp. tomentosoides (van Goor) Silva has spread widely on several temperate-zone, rocky shores where non-weedy conspecific subspecies occur (N.E. Atlantic, N.E. Pacific, S. Pacific). To determine how phenologically and morphologically distinctive the invasive alga was relative to native subspecies, I compared marine intertidal populations of C. fragile ssp. tomentosoides and the native C. fragile ssp. novaezelandiae (J. Ag.) Silva (hereafter referred to as ssp. tomentosoides and ssp. novae-zelandiae respectively on New Zealand shores in 1992, 1993 and 1995. On the North Island, the invasive ssp. tomentosoides is sparsely distributed on low intertidal benches on wave-protected shores in the Hauraki Gulf (east coast) in spring and summer, and thalli die back to the perennial holdfast in autumn. In contrast, the native ssp. novaezelandiae forms dense beds within the low intertidal mussel zone on wave-swept shores of Maori Bay (west coast), and fronds are perennial. Whereas ssp. tomentosoides has only a few fronds arising from the spongy basal hold-fast, ssp. novae-zelandiae thalli are composed of many fronds. The ssp. tomentosoides from the Hauraki Gulf is significantly more branched than comparably sized native conspecifics from Maori Bay. These phenological and morphological differences were used to predict the subspecific identity of C. fragile from three other locations on the North Island, two locations on the South Island, and four locations on S.E. Australian shores; microscopic examination of utricles was used to check the predictions. Seasonality and number of fronds per thallus are the most reliable characters for field identification of native vs invasive subspecies: perennial intertidal thalli with large numbers of fronds are indicative of native subspecies for different geographic regions.