Impact of sewage on the distribution,abundance and community structure of rocky intertidal macro-organisms

The biological effects of a low-volume domestic sewage discharge were studied near Wilson Cove, San Clemente Island, California (USA), from February to June, 1972. There were fewer species and less cover near the outfall (7 macro-invertebrates, 17.6%; 13 macrophytes, 91.7%) than in nearby “unpolluted” control areas (9 macroinvertebrates, 9.2%; 30 macrophytes, 103.4%). The outfall biota was less diverse than that of the controls, as shown by 5 different diversity indices. A great reduction in community stratification (spatial heterogeneity) and, hence, community complexity occurred near the outfall; this reduction in stratification was primarilly due to the absence of Egregia laevigata, Halidrys dioica, Sargassum agardhianum and Phyllospadix torreyi. These were replaced in the mid-intertidal near the outfall by a low turf of blue-green algae, Ulva californica, Gelidium pusillum and small Pterocladia capillacea, and in the lower intertidal by Serpulorbis squamigerus covered with Corallina officinalis Var. chilensis. A statistically-based determination of assemblages or groups of organisms (i.e., cluster analysis) revealed 3 discrete outfall and 3 discrete control area groups; 3 assemblages contained samples from both areas. The distributional patterns of these groups indicate that near the outfall the degree of dilution of discharged sewage is more important in regulating zonation than is tidal height. The enhancement of the suspension feeder Serpulorbis squamigerus and the omnivores Ligia occidentalis, Pachygrapsus crassipes and Anthopleura elegantissima in the outer fringe of the outfall plume hypothetically is due to their ability to utilize sewage as a food source. A critical effect of the outfall may be to decrease environmental stability thereby favoring rapid-colonizers and more sewage-tolerant organisms. The outfall macrophytes were characterized by relatively higher net primary productivities, smaller growth forms, simpler and shorter life histories, and most were components of early successional stages.     

Quantifying effects of pollution on biodiversity: a case study of highly diverse molluscan assemblages in the Mediterranean

Structured sampling designs are important in the assessment of environmental impacts of variable ecological systems. Recent developments have provided a useful framework extending existing univariate techniques into a multivariate context. Measures of taxonomic relatedness have also been introduced, which complement existing measures of diversity of assemblages. In this study, the potential effects of sewage discharge on spatial patterns of highly diverse molluscan assemblages in a Mediterranean rocky subtidal habitat were investigated. Nine 20 cm×20 cm quadrats were taken from each of three sites (80 m–100 m apart) within a putatively impacted location near a sewage outfall (I) and at each of two control locations (Cs) by destructive sampling by SCUBA divers at a depth of 3 m–4 m. A total of 5507 specimens of 151 species were collected. The average and the variance in total abundance of molluscs were greater, on average, at I than at Cs. Higher abundances at the sewage outfall were largely driven by greater numbers of juvenile molluscs. The Shannon diversity of molluscs (H′) was significantly lower at I, but no difference among locations was detected for the total number of species (S). In addition, the taxonomic distinctness (Δ*) of molluscs was greater at Cs, although it was more variable at I. Multivariate analyses showed that there was a significant difference in the structure of assemblages at I compared with Cs. The location near the outfall was characterized by greater abundances of several species, including especially the gastropods Pusillina philippi, Bittium latreilli, and Bittium reticulatum. There was also greater variability in the structure of assemblages among sites and among quadrats at control locations compared to those near the outfall. Using a suite of univariate and multivariate measures, including diversity indices, detailed information on taxonomic structure and analyses of variability at different spatial scales, provided useful insights into the effects of sewage impacts on these diverse assemblages. These results also highlighted the importance of analysing measures of variance, as well as mean in considering effects of stress in natural communities.     

Remarques sur la structuration des populations zooplanctoniques dans la zone de l'émissaire de Marseille-Cortiou

The impact of a polluted water-layer on zooplanktonic populations was studied in the sewage discharge zone at Marseille-Cortiou throughout 1977 and a modification of specific diversity was observed: diversity was instable near the outfall, but increased slightly and became more stable further away in non-polluted stations. The population composition did not seem to change. Species rank varied slightly and the number of species increased with increasing distance from the outfall. Specific dominance remained relatively unchanged, and essentially consisted of only 5 species: Clausocalanus acuicornis, Evadne spinifera, Oithona helgolandica, Temora stylifera and Centropages typicus. The population structure, as illustrated by Frontier diagrams, was higher during or just preceding winter. Population organisation was the same close to or far away from the outfall, although the total population counts and number of species were smaller at the most polluted stations. The taxons exhibiting the largest decreases in density were: euphausids, nauplii, larvae of various brachyurans and gastropods, fish eggs, the pteropod Cavolinia inflexa, the copepod Calanus minor and several Coryceidae. The greatest disturbance of the population structure at all stations was observed during the summer. The composition of the zooplanktonic population seems to depend much more on seasonal factors than on the level of pollution. In was not possible to distinguish a cluster of species which were sufficiently dominant to be considered as really characteristic; perhaps the rather high exposure of the sewage site to winds and waves prevents the formation of a local facies.     

Variability of Water Column Transparency, Volume Flow and Suspended Solids Near San Diego Sewage Outfall (California): 15 Years of Data

The spatial and temporal variability of transparency measured for 15 years at 7 stations near the San Diego sewage outfall has been investigated and compared to the temporal variability of sewage suspended solid discharge and flow rate. the purpose of the time-series analyses was to distinguish natural from human (sewage discharge) causes of temporal changes in transparency. the results show that: 1) variations in transparency are highly correlated over the entire area, but there is a gradient in means and variability in the direction perpendicular to the coast; 2) there are no long term trends for increase or decrease in the water clarity at any of the stations; 3) most of the variance of transparency is contained in the seasonal frequency band; 4) over the same time period sewage discharge has significantly increased and suspended solids decreased; 5) most of the variance of these human-caused properties is in the interannual frequency band; (6) there is no correlation at any time-lag between water clarity and suspended solid discharge or flow. These results lead to the conclusion that these anthropogenic properties are not affecting transparency, while natural factors such as seasonality and distance from coast do.     

Variability of Water Column Transparency,Volume Flow and Suspended Solids Near San Diego Sewage Outfall (California): 15 Years of Data

The spatial and temporal variability of transparency measured for 15 years at 7 stations near the San Diego sewage outfall has been investigated and compared to the temporal variability of sewage suspended solid discharge and flow rate. the purpose of the time-series analyses was to distinguish natural from human (sewage discharge) causes of temporal changes in transparency. the results show that: 1) variations in transparency are highly correlated over the entire area, but there is a gradient in means and variability in the direction perpendicular to the coast; 2) there are no long term trends for increase or decrease in the water clarity at any of the stations; 3) most of the variance of transparency is contained in the seasonal frequency band; 4) over the same time period sewage discharge has significantly increased and suspended solids decreased; 5) most of the variance of these human-caused properties is in the interannual frequency band; (6) there is no correlation at any time-lag between water clarity and suspended solid discharge or flow. These results lead to the conclusion that these anthropogenic properties are not affecting transparency, while natural factors such as seasonality and distance from coast do.     

Community succession following massive ice-scour on a rocky intertidal shore: recruitment,competition and predation during early,primary succession

The importance of species interactions and recruitment variability was examined during the first year and a half of primary succession (1988–1989) on an exposed rocky seashore near Halifax, Nova Scotia. Previous work suggested that emergent rock on these shores is normally dominated by fucoid rockweeds because predatory whelks control the sessile animal competitors, mussels and barnacles, and because herbivorous littorinids control ephemeral algal competitors. Abundances of all species except seasonal ephemeral algae were very small throughout this experiment and we found no significant effects of carnivory, herbivory, plant-animal competition or plant-plant competition. A slight facilitation of Fucus recruitment is attributed to a thin mat of ephemeral, blue-green algae. Very few other studies have directly manipulated intertidal ephemeral algae. As primary succession may be very rare in this assemblage, these results may be specific to these circumstances, but they highlight the varying importance of species interactions with variable recruitment. In particular, it appears that variations in recruitment success may be important to community structure, even when recruitment is not limited by propagule supply. The scale of the study also provides insight into successional processes occurring after the recent, extensive ice-scour of exposed seashores in this region.     

Current and historic distribution and abundance of the inarticulated brachiopod, <Emphasis Type="Italic">Lingula reevii</Emphasis> Davidson (1880), in Kaneohe Bay,Oahu, Hawaii,USA

The inarticulated brachiopod, Lingula reevii Davidson (1880) is a filter-feeding invertebrate that burrows vertically in sandy or mixed sediments. Its only recorded occurrence is from Kaneohe Bay, Oahu, Hawaii, southern Japan, and Ambon, Indonesia. Past surveys of Kaneohe Bay populations suggested a distinct decrease in abundance following the diversion of sewage effluent from the bay in 1978/1979. In the summer of 2004 and 2007, visual surveys were conducted in areas of historical L. reevii abundance as well as in areas appearing to have suitable habitat. In 2004, approximately 2,950 m² at 20 sites within the bay were surveyed using quantitative belt transecting methods. A maximum density of 4 Lingula/m² was observed, a decrease from previous maximum estimates of 500 individuals/m² (Worcester, Dissertation, Zoology Department, University of Hawai′i, pp 49, 1969) and 100 individuals/m² (Emig, J Exp Mar Biol Ecol 52:47–61, 1981). When these 20 sites were revisited in 2007, many had fewer or no L. reevii; therefore, broader scale presence/absence surveys were conducted at 16 additional sites in the bay (also surveyed in 2004). The highest density of L. reevii found in 2007 was 0.94 individuals/m². The continued decline in abundance of L. reevii in Kaneohe Bay may be due, in addition to decreased organic enrichment from diversion of sewage discharge almost 30 years ago, to the more recent reduction of suitable habitat by the invasion of mat-forming alien algae species.     

Inhibition of larval settlement to a soft bottom benthic community by drifting algal mats: An experimental test

The hypothesis that drifting red algal mats inhibit settlement of planktonic larvae was tested in a field experiment in 1986. Substratum free of algae (caged boxes) was compared with substratum covered with algae (natural substratum and open boxes). Whereas settling densities of 1500 to 5500 ind. m^-2 of the bivalves Macoma balthica, Cardium glaucum and Mya arenaria were observed in sediment without algal cover, no recruits of the same species were found beneath the algae during the period of peak settlement (June to July). The same difference was observed for the polychaete Nereis diversicolor, although in this case occasional individuals were found beneath the algae. The results demonstrate that algae mats may be efficient inhibitors of larval settlement to benthic soft-substratum communities. It is concluded that algae act as a larval filter.     

Population dynamics of the sea urchin Strongylocentrotus purpuratus in a Central California kelp forest: recruitment,mortality, growth,and diet

The population dynamics of Strongylocentrotus purpuratus inhabiting dense mats of geniculate coralline algae in a shallow central California Macrocystis pyrifera forest was examined. Sea otters had occupied the area for over two decades. Sea urchin density and size distribution were sampled during 1984–1986 to determine recruitment and mortality patterns. Growth rates were obtained from tetracycline-labeled individuals and changes in sizefrequency modes. Periodic collections were made to examine gut contents and the relationship of diet to food availability. The interaction of recruitment and mortality resulted in a dynamic population structure. Mean densities ranged from 6.5 to 12.7 urchins 0.25 m^-2. The population consisted primarily of urchins up to 40 mm in test diameter in a size distribution which changed from unimodal to bimodal and back over the 2 yr study. Mortality was temporally variable and related to test diameter. Growth rates were somewhat lower than most previous reports for the species and suggested that most of the population was made up of 1 and 2 yr-old individuals. Diet consisted largely of geniculate coralline algae, with fleshy brown algae becoming important when available as drift. This study showed high densities of small urchins can exist in a California kelp forest inhabited by sea otters, but regular recruitment may be necessary to maintain such populations.     

The use of algae to control heavy metals in the environment

Aqueous effluents from a lead mining and milling operation located in southeastern Missouri, USA, caused a degradation of stream quality despite treatment by a large tailings pond. The receiving stream was choked with algal mats which accumulated unexpectedly large amounts of manganese, lead and zinc. A wastewater treatment system was designed to utilize algae and benthic macrophytes to remove metals from the tailings pond effluent. The system has proved successful and water quality in the receiving stream has been improved to drinking water standards.Experiments were conducted to understand more fully the phenomenon of heavy metal accumulation by algae. Radionuclides (²¹⁰Pb,²⁰³Hg,⁶⁵Zn,¹⁰⁹Cd) were used in conjunction with commercially available microculture apparatus to screen several species of algae for heavy metal accumulation. It was found that all species of algae studied concentrated mercury, green algae were more efficient accumulators of cadmium than blue-green algae, one alga (Chlamydomonas) proved best at removing lead from solution and no alga studied removed zinc.     

Spatial patterns and temporal succession in soft-bottom macroinvertebrate assemblages surrounding an ocean outfall on the southern San Diego shelf: relation to anthropogenic and natural events

Spatial patterns and temporal succession in soft-bottom macroinvertebrate assemblages are described for an area of the southern San Diego shelf which is influenced by both anthropogenic and natural events. The study transect covered 19 km along the 60 m-depth contour and spanned the terminus of the Point Loma wastewater outfall (32°40N; 117°17W). Ordination and classification analyses revealed that eight major benthic assemblages occurred in the study area from 1986–1990. These analyses further indicated that: (1) the impact of wastewater discharge on the benthos was asymmetric about the outfall, and (2) primary and secondary impact zones existed within the outfall footprint. The primary impact extended only 0.5 km south, but usually up to 1 km north, of the point of discharge, reflecting the prevailing northerly flow of currents along the San Diego coast. Stations within the primary impact zone usually supported similar faunal assemblages at any point in time. A lessened, or secondary, impact was apparent at stations 1 to 2 km south and 2 km north of the outfall. The two stations 9 km to the north appeared to be outside the outfall footprint. Assemblages in the three zones did not differ greatly in species composition, but did differ with respect to organismal density, relative abundances (dominance hierarchy) of component species, and temporal persistence. Biostimulation of the benthos, manifest as increased species richness and organismal abundance, was apparent in the primary impact zone from 1986–1988, although dominance was relatively low. Dominance increased at these sites during 1989 and 1990 due to large increases in populations of a few small species, such as the polychaete Myriochele sp. M and the ostracod Euphilomedes carcharodonta. Biomass was slightly elevated in the primary impact zone during 1986–1987; however, this trend was not sustained. Communities within the outfall footprint were much less persistent over time than those beyond it. The observed shifts in benthic assemblages within the outfall footprint corresponded to natural oceanographic cycles (i.e., the transition from El Niño to La Niña conditions) and anthropogenic changes (i.e., changes in wastewater output). These observations suggest that the Point Loma outfall has a localized effect on the stability of infaunal communities. Possible destabilization mechanisms are explored. Finally, comparisons with pre-discharge data taken during 1956–1957 revealed that about half of the species present prior to construction of the outfall were still common in 1990, after 27 yr of wastewater discharge, although large increases in species diversity and abundance seem to have occurred at all stations.     

Nitrogen fixation (acetylene reduction) associated with macroalgae in a coral-reef community in the Bahamas

N₂ fixation (C₂H₂ reduction) was associated with several species of macroalgae on a coral reef near Grand Bahama Island. The highest rates were associated with Microdictyon sp. (Chlorophyceae) and Dictyota sp. (Phaeophyceae). Extensive mats of filamentous blue-green algae, not heterotrophic bacteria, were the N₂ fixing agents: in experiments with samples of Microdictyon sp., the activity was lightdependent and not stimulated by organic compounds under either aerobic or anaerobic conditions. Assays in situ, at 20 m depth, and on shipboard, gave similar rates of N₂ fixation; the cyanophytes presumably have pigment adaptations to function in blue light. The maximum rate of N₂ fixation, associated with Microdictyon sp., was 3.8 g N fixed g dry weight^-1 h^-1. Coral-reef communities flourish in nutrientimpoverished waters, and therefore any input of nitrogen is probably important in stabilizing such ecosystems.     

Sewage discharges in oceanic islands: effects and recovery of eulittoral macrofauna assemblages

Sewage discharges are among the most common anthropogenic stressors on rocky shores. However, studies on benthic invertebrates’ responses to sewage pollution on insular assemblages are rare. In order to fulfil this gap, (i) the effects of sewage pollution and (ii) the recovery capacity of eulittoral insular assemblages were examined. The study was conducted on Terceira Island (Azorean Archipelago, Portugal), in four distinct areas: one impacted area (outfall in operation), one post-impacted area (decommissioned outfall) and two undisturbed reference areas. Results showed that the abundance of the dominant species changed between reference and impacted areas. More specifically, the abundance of limpets Patella candei, and barnacles Chthamalus stellatus decreased, while the abundance of littorinids Tectarius striatus and Melarhaphe neritoides increased near the sewage discharges. Seventeen months after removal of the outfall, limpet and barnacle populations had partially recovered, but littorinid populations had still not fully recovered. This study confirms the negative effects of sewage discharges on eulittoral assemblages, and provides information on the resilience of benthic species following the cessation of sewage disposal.     

A kinematic model of the city of Miami ocean outfall plume behavior

A kinematic model is developed to predict the movement of effluent from the City of Miami ocean outfall. The outfall terminates at the 5 m isobath in the near vicinity of three tidal inlets opening into Biscayne Bay. Due to the shallow, uncomplicated topography and oscillatory coastal currents a linear bottom friction law is valid, which justifies the use of a potential flow model. The inlets are modeled as sinks and the outfall as a source discharging into a uniform coastal current. The model predicts that effluent will enter Biscayne Bay in approximately 3 h during flood tide for coastal current headings within an arc of 210° to 40°. Dye-tracking experiments were in good agreement with the model results. Local wind records suggest that sewage effluent enters Biscayne Bay on about 80% of the flood tides, or roughly 11 times per week.     

Fauna of the green alga Cladophora glomerata in the Baltic Sea: density, diversity, and algal decomposition stage

The green alga Cladophora glomerata (L.) is a common macrophyte in the northern Baltic Sea, where it forms drifting mats during summer. We studied the effect of its stage of decomposition on the density, diversity, and resource usage of associated meio- and macrofauna. We hypothesised that mobile species would show small variation in food preferences among decomposition stages, while high variation was expected in stationary species, as reflected in their stable isotope signatures. The assemblage structure of the fauna differed between the 3 studied algal degradation stages, Green (attached, healthy), Degraded (attached, but starting to decay), and Drift (detached, decaying). C/N ratios were highest in green algae and decreased in decaying algal stages. Variation in stable isotope ratios of stationary and mobile species supported our resource use hypothesis. The decomposition stage of C. glomerata significantly affected the carbon and nitrogen stable isotope ratios of both the alga and its main grazer species. Higher invertebrate diversity in the more decaying stages was probably facilitated by decomposer microbes adding resource types and by the proximity of the detached algal mats to the sediment.     

Hepatic mixed-function oxidases in California flatfishes are increased in contaminated environments and by oil and PCB ingestion

Hepatic mixed-function oxidases (MFOs) were measured in the bothid flatfishes Citharicthys sordidus and C. stigmaeus from relatively uncontaminated and from polluted coastal populations of California, USA, at various times of the year during 1979–1980 and in individuals fed crude oil and polychlorinated biphenyl-augmented food in the laboratory. For C. sordidus, aryl hydrocarbon hydroxylase (AHH) specific activity was generally highest around the Los Angeles County sewage outfall on the Palos Verdes Shelf, intermediate near the 7-mile Hyperion sewage outfall in Santa Monica Bay and around a petroleum seep in the Santa Barbara Channel, and lowest in relatively unpolluted Monterey Bay. For C. stigmaeus, which had about ten times less specific activity than the foregoing species, specimens from the Santa Barbara petroleum seep had significantly greater AHH specific activity than those from Monterey Bay. Fishes from contaminated environments also showed increases of microsomal proteins with molecular weights of 56, 54, 57 and 46×10³; moreover, the content of cytochrome P-450 was elevated in specimens of C. sordidus from such environments. Augmentation of food with seep oil or polychlorinated biphenyls (PCBs) induced significant increases in the specific activity of AHH and amounts of microsomal proteins in C. stigmaeus. Thus, these two species of flatfishes are good candidates for monitoring biologically meaningful levels of petroleum and polychlorinated biphenyls in contaminated environments. Moreover, the mixed function oxidase pattern in fish populations from the Santa Barbara petroleum seep is evidently a functional adaptation to chronic intake of petroleum hydrocarbons.     

Nutrient enrichment of seagrass beds in a rhode island coastal lagoon

Seagrass and algal beds showed a variety of reponses when the water column was treated with low level additions of ammonium, nitrate and phosphate. The nutrients were added separately to 3 uniform seagrass beds of a temperature coastal lagoon during 1979 and 1980. (1) Ammonium caused the production of dense mats of free-floating green algae Enteromorpha plumosa and Ulva lactuca. It also stimulated growth in both the leaf and root-rhizome fractions of Zostera marina. This growth response in Z. marina was greater in the area where current reached 12 cm · s^-1 than in the area with little or no current. The concentration of nitrogen in the tissue did not change. In contrast, where current was lacking, Z. marina growth increase with ammonium was small, but the concentration of nitrogen in the tissue doubled over that in control plots. The growth of Ruppia maritima was inversely related to the growth of green algae in the same plots. The red alga Gracilaria tikvahiae did not grow better in ammonium, but its tissue reddened. (2) Nitrate additions enhanced the growth of the green seaweeds Enteromorpha spp. and U. lactuca, but not Z. marina or R. maritima. G. tikvahiae, when fertilized in isolation from other plants, showed a marginal response to this nutrient, and the tissue always reddened. (3) Phosphate enhanced growth in Z. marina and R. maritima exposed to moderate current. G. tikvahiae growing alone showed a small growth response to phosphate. The phosphate made no difference in the growth of the green seaweeds. (4) None of the nutrient supplements noticeably altered the species composition of either epiphytic or planktonic algae associated with the beds, although we did detect small increases in their numbers. The rapid and dense growth of green algae in nitrogen-enriched water probably limited growth of adjacent seagrasses and red algae. Because these seaweeds did not use the phosphate, it became available to other plant components. The overall floral response to nutrient addition in seagrass communities depends, therefore, upon the particular nutrient supplied, the ability of alternate species in the area to compete for that nutrient and the velocity of current in the specific area.     

Système planctonique et pollution urbaine: un aspect des populations zooplanctoniques

Zooplankton populations of a marine area exposed to pollution from the sewage outfall of Marseille-Cortiou have been studied in relation to their horizontal distribution related to distance from the outfall. Samples were collected by various means, and confirmed preliminary results which had indicated that some species tend to cluster in a facies characteristic of a polluted environment. The zone in immediate proximity to the outfall is the most turbid zone; it is extremely poor in fauna but not azoic. Outside this zone and up to 500 m distance from the sewage outlet into the sea, the most euryoecious species (e.g. Acartia clausi, Oithona nana, Euterpina acutifrons, and Oikopleuridae) are frequently present. An intermediate zone then occurs, in which, together with the species mentioned above, other species quite tolerant to pollution (e.g. Clausocalanus spp., Centropages typicus, Paracalanus spp., Candacia armata and some Corycaeidae and Oncaeidae) are well represented. Outside this zone, the zooplanktonic populations show a better defined and more stable structure. Paracalanus spp. are still very abundant, together with Fritillaridae, and fish eggs and larvae. Beneath the turbid surface zone, the impact of pollution seems less marked, and an homogenous zooplankton assemblage is found whose composition is identical to that of species inhabiting unpolluted areas.

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avec la participation du Groupe EPOPEM (Equipe de Pollution Pélagique de Marseille)     

Pit structure and trophic relationship of the coral pit crab Cryptochirus coralliodytes

Individuals of the pit crabs Cryptochirus coralliodytes Heller inhabit massive corals of the family Faviidae. Their pit walls were observed to be covered by blue-green algae and fungi. We suggest that the crabs enhance the growth of these algae and fungi with their metabolic excretions, which contain ammonium. The endolithic algae and the fungi may facilitate the abrasion of the coral skeleton by the crabs, by perforating it and thus weakening the skeletal structure. Computerized tomography analysis revealed dense skeletal material around the pits. Transverse sections showed that the calcification around the pit was similar to other parts of the colony, whereas the macro-architecture was different. Such a difference is the result of the crabs' influence on the corals' living tissue, possibly on the calicoblast which deposits the coenosteum. Crabs, which were exposed to carbon-labeled corals for 1, 7 and 18 d, accumulated labeled carbon, indicating transfer of carbon from the coral tissue to the crabs. Histochemical examination of the stomach and gut of crabs revealed the presence of mucopolysaccharids in the gut, supporting the hypothesis that the crabs eat coral products. The findings of this study provide additional evidence that C. coralliodytes are parasites and support the general hypothesis that a nutritional relationship may have served as a basis for selection. Received: 20 October 1998 / Accepted: 29 April 1999     

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