Gracilaria Tikvahiae Research Articles

Nitrogen allocation of Gracilaria tikvahiae grown in urbanized estuaries of Long Island Sound and New York City, USA: a preliminary evaluation of ocean farmed Gracilaria for alternative fish feeds

Copyright © 2014 The Korean Society of Phycology 227 http://e-algae.kr pISSN: 1226-2617 eISSN: 2093-0860 Nitrogen allocation of Gracilaria tikvahiae grown in urbanized estuaries of Long Island Sound and New York City, USA: a preliminary evaluation of ocean farmed Gracilaria for alternative fish feeds Ronald B. Johnson*, Jang K. Kim, Lisa C. Armbruster and Charles Yarish Resource Enhancement and Utilization Technologies Division, Northwest Fisheries Science Center, National Marine Fisheries Service, 2725 Montlake Blvd. E, Seattle, WA 98112, USA Department of Marine Sciences, University of Connecticut, One University Place, Stamford, CT 06901, USA Department of Ecology and Evolutionary Biology, University of Connecticut, One University Place, Stamford, CT 06901, USA

A Floristic Comparison of Benthic “Marine” Algae In Bras D'Or Lake, Nova Scotia With Five Other Northwest Atlantic Embayments and the Baltic Sea In Northern Europe

The seaweed flora of Bras d'Or Lake, Cape Breton Island, Nova Scotia is compared with those of the Baltic Sea in northern Europe and five Northwest Atlantic embayments ranging from Pomquet Harbour, Nova Scotia to Narragansett Bay, Rhode Island. Both Bras d'Or Lake and the Baltic Sea have experienced extensive geological and hydrographic changes during the past 15,000 years, which have resulted in limited connections to the open sea, negligible tides, low salinities, and reduced floras. The ecology of these two unique saltwater bodies differs from true estuaries like the York River of Maine, the Great Bay Estuary System of New Hampshire/Maine, and the Merrimack River of Massachusetts, which experience extreme hydrographic and tidal variability. Of the 309 total seaweed taxa recorded from the six Northwest Atlantic embayments, Bras d'Or Lake had the lowest number of red, brown, and green algae (i.e., 83) and shared taxa (mean 61.2 ± 4.9 SE), whereas Narragansett Bay had the highest numbers of total (203) and shared taxa (mean 111.6 ± 20.1 SE). Several taxa were restricted to single embayments, with a few glacial and limnic relicts occurring in Bras d'Or Lake and the Baltic Sea. Nine disjunct warm-water taxa were present in Bras d'Or Lake, with this representing the second highest numbers found in four embayments located north of Cape Cod, Massachusetts; this pattern reflects the characteristics of a shallow inland lake. Eleven introduced taxa were recorded from the various Northwest Atlantic embayments, ranging from two in Bras d'Or Lake to 3–10 in the other five embayments. The absence of an intertidal fucoid zone in the Lake (and the Baltic Sea) is probably associated with several physical factors, including a lack of tides, extensive ice-scouring, long-lasting atmospheric reductions of sea level, and reduced salinities. By contrast, the occurrence of subtidal fucoid algae within both brackish water habitats is probably due to a lack of strong competition from perennial kelp populations, which have limited tolerances to low salinities. Morphological features of attached submerged fucoids are analogous to those of loose-lying or entangled salt marsh forms, which are usually small, thin, profusely branched, and have extensive vegetative fragmentation. Unique aegagropilous (detached, ball-shaped) populations of Gracilaria tikvahiae also were found at some inland Bras d'Or Lake sites, presumably resulting from ice scouring and detachment.

Polyculture of the lined seahorse, Hippocampus erectus Perry, 1810 with two species of macroalgae in aquaria

A two-month study was conducted to test the effects of macroalgae on the growth and survivorship of juvenile lined seahorse (Hippocampus erectus) in aquaria. Twenty-day old seahorses were cultured in the tanks with green alga Chaetomorpha linum, red alga Gracilaria tikvahiae, green plastic artificial plant (stargrass), and black plastic net, respectively. Increases in wet mass and standard length for the seahorses were significantly higher in the tanks with macroalgae than those without macroalgae. The seahorse survivorships in the two macroalgal treatments were also significantly higher than those of the two treatments without macroalgae. In the treatments with macroalgae, the DO and pH were higher than those without marcoalgae. During the experiment, the color of seahorses was changed to certain extent. In the green background (the treatments with C. linum and stargrass), 24.1%–28.4% of black seahorses decreased over time; whereas in the black (plastic net) or brown (G. tikvahiae) background treatments, 14.1%–16.3% of yellow seahorses decreased over time, for matching that of the background. Furthermore, the survival rate of seahorse was correlation with DO and pH, and strong correlation with standard length, wet mass and CF. Polyculture with macroalgae, survival rates of seahorse were higher than without macroaglae.

Modeling the role of macroalgae in a shallow sub-estuary of Narragansett Bay, RI (USA)

Proliferation of macroalgal mats is a frequent consequence of nutrient-driven eutrophication in shallow, photic coastal marine ecosystems. These macroalgae have the potential to significantly modify water quality, plankton productivity, nutrient cycling, and dissolved oxygen dynamics. We developed a model for Ulva lactuca and Gracilaria tikvahiae in Greenwich Bay, RI (USA), a shallow sub-estuary of Narragansett Bay, as part of a larger estuarine ecosystem model. The model predicts the biomass of both species in units of carbon, nitrogen, and phosphorus as a function of primary production, respiration, grazing, decay, and physical exchange, with particular attention to the effects of biomass layering on light attenuation and suppression of metabolic rates. The model successfully reproduced the magnitude and seasonal cycle of area-weighted and peak biomass in Greenwich Bay along with tissue C:N ratios, and highlighted the importance of grazing and inclusion of self-limitation primarily in the form of self-shading to overcome an order of magnitude difference in rates of production and respiration. Inclusion of luxury nutrient uptake demonstrated the importance of internal nutrient storage in fueling production when nutrients are limiting. Macroalgae were predicted to contribute a small fraction of total system primary production and their removal had little effect on predicted water quality. Despite a lack of data for calibration and a fair amount of sensitivity to individual parameter values, which highlights the need for further autecological studies to constrain formulations, the model successfully predicted macroalgal biomass dynamics and their role in ecosystem functioning. Our formulations should be exportable to other temperate systems where macroalgae occur in abundance.

Copper, Zinc, Nickel, and Cobalt biosorption potential of Fucus vesiculosus (Phaeophyceae) and Gracilaria tikvahiae (Rhodophyta)

Dried raw and protonated biomasses of the brown seaweed Fucus vesiculosus and the red seaweed Gracilaria tikvahiae were used to study their biosorption performance for copper, zinc, nickel, and cobalt. Representative samples of both species were collected from Chincoteague Bay, Virginia. A series of “sorbate” or metal concentrations (10-450 mg L-1) were selected for each metal to compare the seaweeds' biosorption performance at pH 4.5 ± 0.1 and 25±1 °C. Samples were analyzed on Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-AES). Langmuir and Freundlich adsorption isotherm models for single solute systems were used to assess sorption performance. Both raw and protonated Fucus vesiculosus exhibited higher affinity for metal ions than Gracilaria tikvahiae, at both high and low ends of the concentration ranges. The protonated red seaweed species had maximum metal uptake values of 0.99 mequiv g-1 (copper), 0.39 mequiv g-1 (zinc), 0.66 mequiv g-1 (nickel) and 0.79 mequiv g-1 (cobalt). In comparison, protonated Fucus vesiculosus sorbed higher amounts (1.63, 1.10, 1.40, and 1.69 mequiv g-1 of the metals in the same order). The prospects of using the brown seaweed species in the treatment of industrial wastewaters in place of expensive, conventional ion-exchange resins are envisaged.

Dietary Shifts by Green Turtles (Chelonia mydas) in the Kāne‘ohe Bay Region of the Hawaiian Islands: A 28-Year Study

The green turtle, Chelonia mydas, has modified its feeding behavior to include the increasing abundance of nonnative algae growing in the greater Kāne‘ohe Bay area of O‘ahu in the Hawaiian Islands. Changes in diet of the green turtle are correlated with an increase in abundance of seven species of nonnative algae between 1977 and 2005. Turtles were found to be eating 130 species of marine vegetation, and the three most common were the nonnative species Acanthophora spicifera, Hypnea musciformis, and Gracilaria salicornia. These three abundant and nutritious food sources are now an important part of the turtle diet in addition to native species found in and near Kāne‘ohe Bay. Chelonia mydas behavior has shifted to include these new seaweeds within 10 years of their introduction to the region. The turtles have also gradually included an additional four less-prolific slow-growing nonnative algal species (Eucheuma denticulatum, Gracilaria tikvahiae, Kappaphycus striatum, and Kappaphycus alvarezii), but the time it has taken turtles to include these species has been longer, 20–30 years, after the seaweeds were introduced. During this same 28-year time period numbers of C. mydas have increased throughout the Hawaiian Islands.

Macrophyte Abundance in Waquoit Bay: Effects of Land-Derived Nitrogen Loads on Seasonal and Multi-Year Biomass Patterns

Anthropogenic inputs of nutrients to coastal waters have rapidly restructured coastal ecosystems. To examine the response of macrophyte communities to land-derived nitrogen loading, we measured macrophyte biomass monthly for 6 years in three estuaries subject to different nitrogen loads owing to different land uses on the watersheds. The set of estuaries sampled had nitrogen loads over the broad range of 12 to 601 kg N ha−1 year−1. Macrophyte biomass increased as nitrogen loads increased, but the response of individual taxa varied. Specifically, biomass of Cladophora vagabunda and Gracilaria tikvahiae increased significantly as nitrogen loads increased. The biomass of other macroalgal taxa tended to decrease with increasing load, and the relative proportion of these taxa to total macrophyte biomass also decreased. The seagrass, Zostera marina, disappeared from the higher loaded estuaries but remained abundant in the estuary with the lowest load. Seasonal changes in macroalgal standing stock were also affected by nitrogen load, with larger fluctuations in biomass across the year and higher minimum biomass of macroalgae in the higher loaded estuaries. There were no significant changes in macrophyte biomass over the 6 years of this study, but there was a slight trend of increasing macroalgal biomass in the latter years. Macroalgal biomass was not related to irradiance or temperature, but Z. marina biomass was highest during the summer months when light and temperatures peak. Irradiance might, however, be a secondary limiting factor controlling macroalgal biomass in the higher loaded estuaries by restricting the depth of the macroalgal canopy. The relationship between the bloom-forming macroalgal species, C. vagabunda and G. tikvahiae, and nitrogen loads suggested a strong connection between development on watersheds and macroalgal blooms and loss of seagrasses. The influence of watershed land uses largely overwhelmed seasonal and inter-annual differences in standing stock of macrophytes in these temperate estuaries.

Heavy metal biomonitoring by seaweeds on the Delmarva Peninsula, east coast of the USA

Abstract Metal pollution in aquatic ecosystems can pose a major environmental concern impacting various life-forms. The usefulness of seaweeds as biomonitors or indicators of metals in the coastal bays of the Delmarva Peninsula is highlighted in this study, conducted in early spring 2006. Lack of any preexisting data on metal loads in Delmarva seaweeds and in the ambient medium (water and sediments) led to this pilot study for preliminary investigations on the ecological health of these coastal bays. The seaweed species collected from three different sites of the Delmarva Peninsula were: phaeophytes Fucus vesiculosus, and Fucus distichus, chlorophytes Ulva lactuca, and Enteromorpha intestinalis, and rhodophyte Gracilaria tikvahiae. The sampled seaweeds, seawater, and sediments were subsequently analyzed for chromium, manganese, cobalt, nickel, copper, lead, arsenic, and zinc. The results showed high concentrations of Mn, and low concentrations of Cu in almost all the species. Bioconcentration factors (BCFs) in Ulva lactuca and Fucus vesiculosus, across sites, were similar (though the orders of magnitude varied) for Cu, Zn, and Ni. Calculation of metal pollution index (MPI) and Tomlinson pollution load index (PLI) showed that all three sites had low contamination with the metals investigated.

PHYLOGEOGRAPHY OF GRACILARIA TIKVAHIAE (GRACILARIACEAE, RHODOPHYTA): A STUDY OF GENETIC DISCONTINUITY IN A CONTINUOUSLY DISTRIBUTED SPECIES BASED ON MOLECULAR EVIDENCE1

Gracilaria tikvahiae, a highly morphologically variable red alga, is one of the most common species of Gracilariaceae inhabiting Atlantic estuarine environments and the Intracoastal Waterway of eastern North America. Populations of G. tikvahiae at the extremes of their geographic range (Canada and southern Mexico) are subjected to very different environmental regimes. In this study, we used two types of genetic markers, the chloroplast‐encoded rbcL and the nuclear internal transcribed spacer (ITS) region, to examine the genetic variability within G. tikvahiae, for inferring the taxonomic and phylogenetic relationships between geographically isolated populations, and to discuss its distributional information in a phylogeographic framework. Based on rbcL and ITS phylogenies, specimens from populations collected at the extreme distributional ranges reported for G. tikvahiae are indeed part of the same species; however, rbcL‐ but not ITS‐based phylogenies detected phylogenetic structure among the ten G. tikvahiae different haplotypes found in this study. The four distinct rbcL lineages were identified as 1) a Canadian–northeast U.S. lineage, 2) a southeast Florida lineage, 3) an eastern Gulf of Mexico lineage, and 4) a western Gulf of Mexico lineage. We found no evidence for the occurrence of G. tikvahiae in the Caribbean Sea. Observed phylogeographic patterns match patterns of genetic structures reported for marine animal taxa with continuous and quasicontinuous geographic distribution along the same geographic ranges.

PHYLOGENY AND SYSTEMATICS OF THE MARINE ALGAL FAMILY GRACILARIACEAE (GRACILARIALES, RHODOPHYTA) BASED ON SMALL SUBUNIT rDNA AND ITS SEQUENCES OF ATLANTIC AND PACIFIC SPECIES1

We sequenced the small subunit rDNA and internal transcribed spacer region of Gracilariaceae from the tropical Atlantic and Pacific, with emphasis on flattened or compressed species. Sequence comparisons confirmed three main lineages of Gracilariaceae: Curdiea/Melanthalia, Gracilariopsis/Gracilariophila, and Gracilaria. The Curdiea/Melanthalia diverged early in the family. Gracilariopsis was paraphyletic, because at least one Gracilariophila species evolved from it. The Atlantic Gracilariopsis were monophyletic and separated from the Pacific lineages. The Gracilaria included all species referable to its own species and to Hydropuntia, which was paraphyletic, formed by distantly related lineages. The new combination Gracilaria pauciramosa (N. Rodriguez Rios) Bellorin, M. C. Oliveira et E. C. Oliveira is proposed for Polycavernosa pauciramosa N. Rodriguez Rios. Recognition of subgenera within Gracilaria, based on spermatangial arrangement, was not supported. Instead, infrageneric groups were delineated by geographic origins and combinations of reproductive characters. Most Pacific species with either “textorii” or “verrucosa” type spermatangia were deeply separated from Atlantic species. Within the Atlantic Gracilaria, a lineage encompassing mostly tropical cylindrical species with “henriquesiana” type spermatangia and distinctive cystocarp anatomy was recognized. A lineage was also retrieved for cold water stringy species with verrucosa type spermatangia. Several species from the western Atlantic are closely related to Gracilaria tikvahiae McLachlan with nearly identical morphology. On the other hand, most flattened species from the tropical Atlantic were closely related despite their diverse morphologies. The interpretation of our data in addition to the literature indicates that more populations from the Indo-Pacific must be studied before a general picture of Gracilariaceae evolution can be framed.

Characterization of Intensive Shrimp Pond Effluent and Preliminary Studies on Biofiltration

Abstract Effluent from aquaculture ponds may add to receiving waters high levels of biochemical oxygen demand (BOD), inorganic and organic particulate matter, live algae, dissolved organic matter, ammonia, nitrite, nitrate, phosphate, and other potential contaminants. We evaluated a method that used the eastern oyster Crassostrea virginica and a macroalga, Gracilaria tikvahiae, for biofiltering the effluent from a pond used for intensive culture of Pacific white shrimp Litopenaeus vannamei.2 Effluent flow rates of 2.0, 1.1, and 0.2% of pond volume/d were tested. Shrimp yield from the 0.5-ha pond after 129 d was 10,312 kg/ha; shrimp survival rate was 96.5%. Oyster biofiltration accounted for most of the decrease of turbidity, chlorophyll a, total suspended solids (TSS), and settleable solids. Following macroalgal biofiltration, decreases in ammonia, total nitrogen, and total phosphorus concentrations were observed. At a daily exchange rate of 7.5%, diversion of 1.1% of the pond volume to the treatment syst...

Effects of nitrogen load and irradiance on photosynthetic pigment concentrations in Cladophora vagabunda and Gracilaria tikvahiae in estuaries of Waquoit Bay.

If coastal zone managers wish to regulate septic nitrogen loads, they could concentrate on management of septic systems that lie within 480-730 m of the shore, since these appeared to be the major contributors of nitrate to receiving estuaries. Septic sources farther upgradient probably contribute less significantly. This research was supported by an internship from the Woods Hole Marine Science Consortium to Elizabeth Westgate, an MIT Sea Grant (#65591) awarded to Ivan Valiela and Harold Hemond, and National Estuarine Research Reserve Fellowships to Kevin Kroeger and Wendy Pabich.

Selection of Nitrogen-Enriched Macroalgae (Cladophora vagabunda and Gracilaria tikvahiae) by the Herbivorous Amphipod Microdeutopus gryllotalpa.

Previous articleNext article No AccessECOLOGYSelection of Nitrogen-Enriched Macroalgae (Cladophora vagabunda and Gracilaria tikvahiae) by the Herbivorous Amphipod Microdeutopus gryllotalpaE. Galan Jimenez, J. Hauxwell, E. Heckscher, C. Rietsma, and I. ValielaE. Galan Jimenez Search for more articles by this author , J. Hauxwell Search for more articles by this author , E. Heckscher Search for more articles by this author , C. Rietsma Search for more articles by this author , and I. Valiela Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Biological Bulletin Volume 191, Number 2October 1996 Published in association with the Marine Biological Laboratory Article DOIhttps://doi.org/10.1086/BBLv191n2p323 Views: 35Total views on this site Copyright © 1996 by Marine Biological LaboratoryPDF download Crossref reports no articles citing this article.

Contribution by Macroalgal Mats to Primary Production of a Shallow Embayment Under High and Low Nitrogen-loading Rates

The limits on primary production in areas undergoing eutrophication may be set by indirect effects of nitrogen loading, i.e. decreasing irradiances, associated with proliferating opportunistic algae. Using in situphoton flux density (PFD) availability estimates within unattached algal mats and photosynthetic parameters determined from photosynthesis vs.irradiance (P vs.I) curves generated for the dominant components of mat assemblages, Cladophora vagabundaand Gracilaria tikvahiae, seasonal net mat production rate for estuaries (Waquoit Bay, Massachusetts, U.S.A.) receiving high (Childs River) and low (Sage Lot Pond) N-loading rates were determined. Although abundance of C. vagabundawas 2× greater than G. tikvahiae, the former species contributed only about 50% of total mat productivity, due largely to rapid light attenuation within the dense algal mat. While mat production was low and similar at both sides during winter (≈0·35 g C m −2 day −1), for other seasons, the net mat daytime productivity at Childs River, the N-loaded site, was 2·5× higher than rates determined for Sage Lot Pond. Although annual net daytime production at Childs River (1094 g C m −2 year −1) was comparable to estimates for other algal mat assemblages in eutrophic systems, primary production of the Waquoit Bay system was found to become self-limiting as available PFD controls maximum productivity. In photosynthetically inactive portions of the algal mat, carbon release was estimated from tissue-loss measurements at 0·14 g C m −2 day −1for C. vagabundaand 0·05 g C m −2 day −1for G. tikvahiae. Annual in situC release of 73 g C m −2is ≈20% of annual net mat production (fixed carbon not respired by algae) in this embayment. Although both estuaries showed net autotrophy year round (Pg:R>1), the high metabolic cost of a large, inactive mat resulted in lower Pg:R ratios at Childs Rivers than at Sage Lot Pond, particularly during the summer period of peak production. Thus, it is predicted that the Waquoit Bay system will experience an overall decline in Pg:R ratios and consequent increase in anoxic events as eutrophication continues.

Utilization of microbial organic aggregates by bay scallops, Argopecten irradians (Lamarck)

The use of organic aggregates as a food source by bay scallops, Argopecten irradians (Lamarck), was quantified and compared with the use of phytoplankton, a known good food. In two experiments, the first designed to characterize nitrogen incorporation and the second nitrogen release, 15N was used to trace N into and out of scallops under defined, laboratory conditions. Scallops were fed experimental diets of labelled organic aggregates, produced from the dissolved material released by either a brown ( Fucus vesiculosis) or red ( Gracilaria tikvahiae) alga, or phytoplankton, Thalassiosira weissflogii (Grunow), for periods of 5–15 h in a flow-through system. More N was incorporated by scallops fed phytoplankton (523 μg · gDW −1 · h −1) than by those fed aggregates (70 μg · gDW −1 · h −1 for aggregates derived from F. vesiculosis and 306 μg · gDW −1 · h −1 for those derived from G. tikvahiae). On both diets, fecal material and 15N was released rapidly during the first 6 h of depuration. It was estimated that scallops incorporated aggregate N with an absorption efficiency of 77 to 79% as compared with an efficiency of 90% for phytoplankton N. Estimated assimilation efficiency ranged from 14 to 43% for scallops fed aggregates, and 75% for those fed phytoplankton. Despite these differences, aggregates represent a potentially important source of nutrition for suspension-feeding bivalves living in near-shore regions rich in natural seston.

Agar Analysis, nuclear genome quantification and characterization of four agarophytes (Gracilaria) from the Mexican Gulf Coast

DNA reassociation kinetics were used to determine nuclear genome organization and complexity in four species of Gracilaria (Gracilariales, Rhodophyta). In Gracilaria tikvahiae, G. caudata, G. cervicornis and G. divaricata, results indicate the presence of three second order components corresponding to fast, intermediate and slow fractions. Repetitive sequences varied from 13–46% and unique DNA ranged from 45–78%, Thermal denaturation (Tm) indicated guanine + cytosine (G + C) levels of 41.9–46.0 mol % G + C. Microspectrophotometry with the DNA-localizing fluorochrome DAPI was used to quantify nuclear DNA content. Comparisons of mean nuclear DNA (If) values to chicken erythrocytes (RBC) resulted in an estimate of 0.37–0.40 pg/2C genomes for the four Gracilaria species. Total agar content following alkaline pretreatment ranged from 7–15% dry weight. Gel strengths were generally below commercial levels, ranging from 40–260 g cm−2 Nuclear genome profiles developed from information for genome size, organization and complexity are compared with data for agar quantity and quality. Gel quality and quantity do not appear to be correlated with either large repetitive fraction DNA or a high degree of genome complexity as previously speculated.

Physiological responses of the opportunistic macroalgae Cladophora vagabunda (L.) van den Hoek and Gracilaria tikvahiae (McLachlan) to environmental disturbances associated with eutrophication

Ability of macroalgae to tolerate environmental disturbances often contributes to their success in marine communities. We investigated the effects of episodic and chronic disturbances (hypoxia/anoxia, elevated ammonium, reduced light) associated with eutrophication on physiological responses (respiration, photosynthesis, growth, nutrient uptake) of the macroalgae, Cladophora vagabunda (L.) van den Hoek and Gracilaria tikvahiae (McLachlan). These opportunistic species accumulate into thick (0.3 to > 1 m) mats during summer in a eutrophic embayment (Waquoit Bay, Massachusetts), resulting in a highly reducing environment characterized by elevated ammonium concentrations. Elevated (100 μM) ammonium levels depressed growth and ammonium uptake rates of both species. In fact, C. vagabunda had high rates of ammonium release under conditions similar to the in-mat environment (dark, elevated N). Ammonium uptake rate was 2–3 times higher for G. tikvahiae under lighted vs. dark conditions. This light/dark effect was more dramatic for C. vagabunda; presence of light, characteristic of the surface of the mat, substantially reduced the inhibitory effect of the elevated ammonium treatment. In the field and laboratory, C. vagabunda and G. tikvahiae showed reduced respiration rates under stagnant conditions and hypoxia, respectively. As hypoxic, stagnant conditions occur frequently within the thick mats, a reduction in respiration may enable these species to survive these disturbances. We also investigated the potential for recovery of algal thalli buried within the anoxic region of the mat. C. vagabunda demonstrated remarkable resilience; following a 2-day acclimation period under lighted, aerated conditions, blackened fronds taken from anoxic regions of the mat showed nearly full physiological recovery, namely, photosynthetic performance and nutrient uptake rates approached values measured for surface-collected fronds. Previous field measurements indicated that while C. vagabunda is capable of surviving long-term burial (> 3 months) within the mat, turnover of the mat is a frequent event during summer due to heavy motor-boat activity. The distinctive species' patterns may be critical acclimation responses to large swings in the oxygen and nutrient environments of the algal mat, contributing to the successful dominance of both Cladophora spp. and G. tikvahiae in this and other areas undergoing eutrophication.

COMPETITIVE INTERACTIONS BETWEEN THE OPPORTUNISTIC MACROALGAE CLADOPHORA VAGABUNDA (CHLOROPHYTA) AND GRACILARIA TIKVAHIAE (RHODOPHYTA) UNDER EUTROPHIC CONDITIONS1

ABSTRACT In a eutrophic embayment (Waquoit Bay, Massachusetts), Cladophora vagabunda (L.) van den Hoek occurs in thick (sometimes > 1 kg dry wt ‐m−2), nearly monospecific unattached mats in deeper regions (2 m), whereas Grac‐ilaria tikvahiae McLachlan is largely restricted to shallow (<1 m) areas. We explored these distribution patterns, investigating competitive interactions between these opportunistic species by varying the limiting resource, photon flux density (PFD), and species composition under conditions of N sufficiency in microcosms. Under lower biomass loadings, neither species showed a difference in growth rates in single‐ and mixed‐species stands. With a 25% increase in initial biomass loading, Gracilaria tikvahiae had significantly higher growth rates under saturating PFD and consistently showed greater performance when grown in single‐species rather than in mixed‐species stands. While growth rate was 2×greater for C. vagabunda in single‐species than in mixed‐species stands at saturating PFD, this pattern was reversed under limiting irradi‐ances. In mixed‐species stands at high PFD (comparable to shallow regions of the bay), the growth rate of G. tikvahiae was over 4×higher than that of C. vagabunda. Cladophora vagabunda grew at a faster rate than G. tikvahiae only in the low PFD, mixed‐species treatment. Results of this study suggest that the observed distributional patterns of these macroalgae are due in part to interspecific exploitative competition but that tolerance of low PFD by C. vagabunda has led to dominance of these species in distinctive regions of the embayment.

SUMMER DECLINE OF ULVA LACTUCA (CHLOROPHYTA) IN A EUTROPHIC EMBAYMENT: INTERACTIVE EFFECTS OF TEMPERATURE AND NITROGEN AVAILABILITY?1

ABSTRACT Throughout the summer, abundance of Ulva lactuca L. declined while biomass of Cladophora vagabunda (L.) van den Hoek and Gracilaria tikvahiae McLachlan increased in a New England embayment undergoing eu‐trophication (Waquoit Bay, Massachusetts). We investigated the physiological basis for the summer dieback, focusing on temporal variations in photosynthetic performance and tissue nitrogen (N). We also compared photosynthetic and N uptake capabilities of U. lactuca with other abundant species in this eutrophic system. Photosynthetic egiciency and capacity of U. lactuca declined markedly at 25°C, compared with a spring (15°C) peak in photosynthetic performance; Pmax was 4.6 ± 0.3 and 1.8 ± 0.6 μmol O2.m−2.s−1 during spring and summer, respectively. Notably, summer pmax of other abundant species of the embayment was 1.5–3 × higher than that measured for U. lactuca. Ulva lactuca showed a signifciant photosynthetic response to dissolved inorganic carbon enrichment during summer, when water‐column‐dissolved CO2 levels were 20% of spring values. Although ammonium uptake rates of U. lactuca were extremely high at both subsaturating (15μM) and saturating (75 μM) N concentrations, as predicted by the functional‐form hypothesis, tissue N fell to 1% by late summer. We suggest that a carbon imbalance, initiated by rising water temperatures and declining water‐column N; thermal stress; and biological factors (competition, grazing) all contribute to the recurrent summer decline of U. lactuca in this shallow, eutrophic embayment. Thus, while the morphology of U. lactuca might be considered a successful strategy for disturbed, or “stressed” (sensu Littler and Littler 1980), habitats, its inability to persist and flourish in this environment emphasizes the complexity of factors at work in natural systems.

Interactive effects of nitrogen and dissolved inorganic carbon on photosynthesis, growth, and ammonium uptake of the macroalgae Cladophora vagabunda and Gracilaria tikvahiae

Dissolved inorganic carbon (DIC) is rarely considered limiting for macroalgae, but some research suggests that under conditions of N sufficiency, photosynthetic capacity is enhanced with DIC enrichment. During spring (April–May) and summer (July–August) 1993, we investigated the interactive effects of nitrogen (N) and DIC on photosynthetic capacity, growth, and nutrient uptake rates of the macroalgae, Cladophora vagabunda (L.) van den Hoek and Gracilaria tikvahiae (McLachlan), dominant species in a temperate eutrophic estuary (Cape Cod, Massachusetts, USA). Water-column CO2 concentrations showed significant diurnal fluctuations, ranging from a morning CO2 peak (21 μM) to an afternoon low (13 μM) during summer, probably associated with metabolic activities in a thick algal mat. Results from instantaneous photosynthesis measurements and microcosm experiments indicate that DIC limits photosynthetic capacity and growth rates of C. vagabunda during summer, perhaps related to tissue N sufficiency and low water-column CO2 concentrations. For example, this species showed enhanced growth (F=8.69, P<0.02) under DIC but not N enrichment. G. tikvahiae showed marginal DIC enhancement of maximum photosynthetic rate, while growth was significantly stimulated by addition of N. Reduced thallus N of this species during the summer further identifies N as the primary factor limiting growth. In addition, G. tikvahiae has the ability to use DIC in its several forms, while C. vagabunda primarily uses dissolved CO2. DIC enrichment resulted in a depression of NH4+ uptake rates for both species, particularly during summer at saturating (60 μM) ammonium levels, suggesting competition between NH4+ uptake and DIC acquisition under conditions of N sufficiency. Dominance of C. vagabunda and G. tikvahiae in areas undergoing eutrophication has been attributed to their successful procurement and storage of N as well as to high growth rates. The present study revealed that under conditions of N sufficiency during summer, DIC may control rates of production of these opportunistic macroalgae.