Effects of nutrients, herbivory, and depth on the macroalgal community in the rocky sublittoral

We studied the interacting roles of nutrient availability and herbivory in determining the macroalgal community in a rocky littoral environment. We conducted a factorial field experiment where we manipulated nutrient levels and herbivory at two sublittoral depths and measured macroalgal colonization and the following young assemblage during the growing season. At the community level, grazing reduced algal colonization, though the effect varied with depth and its interaction with nutrient availability varied in time. In shallow water, the total density of macroalgae increased in response to nutrient enrichment, but the ability of grazers to reduce macroalgal density also increased with the nutrient enrichment, and thus, the community could not escape from the top-down control. In deep water, the algal density was lower, except in July when nutrient enrichment caused a very dense algal growth. Grazing at the greater depth, though effective, was generally of smaller magnitude, and in July it could not limit algal recruitment and growth. Species richness peaked at the intermediate nutrient level in deep but not in shallow water during most of the growing season. Grazing had no effect on diversity of the algal community at either depth and only a minor effect on species richness at the greater depth. Opportunistic and ephemeral algae benefited from the nutrient enrichment but were also grazed to very low densities. Slowly growing and/or perennial species colonized poorly in the nutrient enriched treatments, and depending on the species, either suffered or indirectly benefited from herbivory. For all species, effects of nutrients on colonization depended on depth; usually both nutrient and herbivory effects were more pronounced at the shallow depth. We conclude that grazers are able to reduce macroalgae over a large range of nutrient availabilities, up to 12-fold nutrient enrichment in the current experiment, and that the sublittoral depth gradient generates variation in the algal community control exerted by both herbivory and nutrient availability. Thus temporal and spatial variability in both top-down and bottom-up control and in their interaction, especially along the depth gradient, may be crucially important for producer diversity and for the successional dynamic in a rocky sublittoral environment.     

Wind induced upwelling as a possible explanation for mass occurrences of epiphyticEctocarpus siliculosus (Phaeophyta) in the northern Baltic Proper

The quantity of epiphytic filamentous algae varies considerably in macroalgal vegetation along rocky shores of the northern Baltic Proper. The main species responsible for irregular mass occurrences is the summer annual brown algaEctocarpus siliculosus (Dillw.) Lyngb. In this study, data collected over a 3-yr monitoring period are related to hydrographical parameters. The tideless and brackish Baltic Sea is salinity stratified, and the salinity difference between surface and bottom waters can be used to indicate upwelling events. Mass occurrences developed when the salinity difference was low. These salinity differences explained 87% of the observed increase inE. siliculosus cover. We propose that the slight salinity changes themselves do not affect the growth ofE. siliculosus, but rather they can be used as indicators of short nutrient pulses. These nutrient pulses are not detected in standard water sampling, as they are utilised by both pelagic and benthic organisms within a few hours or days. If the proposed mechanism is true, changes in wind speed and direction have an important effect on the nutrient dynamics of littoral algal communities. A higher frequency of SW winds may cause mass occurrences of filamentous algae, which are often believed to indicate eutrophication of an anthropogenic origin.     

Biotic habitat complexity controls species diversity and nutrient effects on net biomass production

Canopy-forming plants and algae commonly contribute to spatial variation in habitat complexity for associated organisms and thereby create a biotic patchiness of communities. In this study, we tested for interaction effects between biotic habitat complexity and resource availability on net biomass production and species diversity of understory macroalgae by factorial field manipulations of light, nutrients, and algal canopy cover in a subtidal rocky-shore community. Presence of algal canopy cover and/or artificial shadings limited net biomass production and facilitated species diversity. Artificial shadings reduced light to levels similar to those under canopy cover, and net biomass production was significantly and positively correlated to light availability. Considering the comparable and dependent experimental effects from shadings and canopy cover, the results strongly suggest that canopy cover controlled net biomass production and species diversity by limiting light and thereby limiting resource availability for community production. Canopy cover also controlled experimental nutrient effects by preventing a significant increase in net biomass production from nutrient enrichment recorded in ambient light (no shading). Changes in species diversity were mediated by changes in species dominance patterns and species evenness, where canopy cover and shadings facilitated slow-growing crust-forming species and suppressed spatial dominance by Fucus vesiculosus, which was the main contributor to net production of algal biomass. The demonstrated impacts of biotic habitat complexity on biomass production and local diversity contribute significantly to understanding the importance of functionally important species and biodiversity for ecosystem processes. In particular, this study demonstrates how loss of a dominant species and decreased habitat complexity change the response of the remaining assembly to resource loading. This is of potential significance for marine conservation since resource loading often promotes low habitat complexity and canopy species are among the first groups lost in degraded aquatic systems.     

Selective consumption and facilitation by mesograzers in adult and colonizing macroalgal assemblages

Grazing pressure on macroalgae in littoral communities may vary with algal species, the age of an algal individual and grazer identity. Previous studies on alga–grazer interactions have shown that grazer preference for an algal species may release another one from interspecific competition. We measured the impacts of four common grazer taxa and the natural grazer guild on macroalgal communities at both their colonization and adult stages, and compared the impacts to grazer exclosures. The grazer effects were stronger on colonizing than on adult macroalgae; grazers did not reduce the total density of adult algae. Grazers both feed on propagules and indirectly facilitate other algae, depending on the grazer or algal species. Hydrobia species increased the settlement of spores of the red alga Ceramium tenuicorne. Similarly, the gastropod Theodoxus fluviatilis tended to facilitate one crustose algal species, but decreased the propagule density of annual filamentous algae, suggesting a preference for one species to the advantage of another. Effects of crustacean mesograzers on the studied macroalgae were weak. These results indicate that northern Baltic macroalgae are limited to grazing mainly during their colonization stage.     

Effects of micro- and mesograzers on intertidal macroalgal recruitment

This study examined the effects of a guild of micrograzing harpacticoid copepods (dominated by two species of Paradactylopodia sp. nov. and one species of Scutellidium sp. nov.) and a mesograzing periwinkle, Afrolittorina praetermissa, on the early recruitment of intertidal macroalgae on a wave-exposed, rocky shore. This is the first study, to our knowledge, to examine the effects of micrograzers (<500 μm) on intertidal macroalgal recruitment. Data showed that microscopic harpacticoid copepods altered the assemblages and reduced the densities of several macroalgal taxa, while A. praetermissa changed the assemblages and reduced both the density and number of macroalgal taxa. Recruitment of encrusting coralline algae was actually higher in copepod inclusions than exclusions, suggesting that copepods may be beneficial to the recruitment of this algal group. These results contribute to the understanding of grazing as a factor causing high mortality of algal recruits, but also highlight the need for more studies that examine the effects of micro- and mesograzers on the distribution and abundance of macroalgae.     

Food web structure of a restored macroalgal bed in the eastern Korean peninsula determined by C and N stable isotope analyses

Loss of macroalgae habitats has been widespread on rocky marine coastlines of the eastern Korean peninsula, and efforts for restoration and creation of macroalgal beds have increasingly been made to mitigate these habitat losses. Deploying artificial reefs of concrete pyramids with kelps attached has been commonly used and applied in this study. As a part of an effort to evaluate structural and functional recovery of created and restored habitat, the macroalgal community and food web structure were studied about a year after the establishment of the artificial macroalgal bed, making comparisons with nearby natural counterparts and barren ground communities. Dominant species, total abundance, and community structure of macroalgal assemblage at the restored macroalgal bed recovered to the neighboring natural bed levels during the study period. The main primary producers (phytoplankton and macroalgae) were isotopically well separated. δ¹³C and δ¹⁵N values of consumers were very similar between restored and natural beds but varied greatly among functional feeding groups. The range of consumer δ¹³C was as wide as that of primary producers, indicating the trophic importance of both producers. There was a stepwise trophic enrichment in δ¹⁵N with increasing trophic level. A comparison of isotope signatures between primary producers and consumers showed that, while suspension feeders are highly dependent on pelagic sources, invertebrates of other feeding guilds and fishes mainly use macroalgae-derived organic matter as their ultimate nutritional sources in both macroalgal beds, emphasizing the high equivalency of trophic structure between both beds. Isotopic signatures of a few mollusks and sea urchins showed that they use different dietary items in macroalgal-barren grounds compared with macroalgal beds, probably reflecting their feeding plasticity according to the low macroalgal biomass. However, isotopic signatures of most of the consumers at the barren ground were consistent with those at the macroalgal beds, supporting the important trophic role of drifting algae. Our results revealed the recoveries of the macroalgal community and trophic structure at the restored habitat. Further studies on colonization of early settlers and the following succession progress are needed to better understand the process and recovery rate in the developing benthic community.     

Microhabitat segregation of the amphipod genus Gammarus (Crustacea: Amphipoda) in the Northern Baltic Sea

Closely related species may occupy very similar niches but are often found to diverge by one or more traits when they share the same habitat. Five indigenous and sympatric Gammarus species are characteristic for the Baltic rocky littoral ecosystem. Yet, the species-specific distribution of these sympatric gammarids has not been well studied in the northern Baltic Sea. This study was undertaken to assess the spatial distribution of gammarid amphipods along wave exposure and depth gradients to study whether they show segregation in their microhabitat use. We sampled 12 rocky sublittoral shores along a wave exposure gradient over a period of 5 years. Samples differed with respect to depth and macroalgal type. Three of the five gammarid species occurred mainly in different depth zones and among different macroalgae at the exposed shores. In contrast, on protected shores, where algal zonation is weaker, a link to zonation and macroalgal type was almost absent. Moreover, the microhabitat use was strongest during the reproductive seasons of the species. The observed microhabitats of the three gammarid species fit well to their species-specific mean body sizes.     

Herbivore vs. nutrient control of marine primary producers: context-dependent effects

Pervasive overharvesting of consumers and anthropogenic nutrient loading are changing the strengths of top-down and bottom-up forces in ecosystems worldwide. Thus, identifying the relative and synergistic roles of these forces and how they differ across habitats, ecosystems, or primary-producer types is increasingly important for understanding how communities are structured. We used factorial meta-analysis of 54 field experiments that orthogonally manipulated herbivore pressure and nutrient loading to quantify consumer and nutrient effects on primary producers in benthic marine habitats. Across all experiments and producer types, herbivory and nutrient enrichment both significantly affected primary-producer abundance. They also interacted to create greater nutrient enrichment effects in the absence of herbivores, suggesting that loss of herbivores produces more dramatic effects of nutrient loading. Herbivores consistently had stronger effects than did nutrient enrichment for both tropical macroalgae and seagrasses. The strong effects of herbivory but limited effects of nutrient enrichment on tropical macroalgae suggest that suppression of herbivore populations has played a larger role than eutrophication in driving the phase shift from coral- to macroalgal-dominated reefs in many areas, especially the Caribbean. For temperate macroalgae and benthic microalgae, the effects of top-down and bottom-up forces varied as a function of the inherent productivity of the ecosystem. For these algal groups, nutrient enrichment appeared to have stronger effects in high- vs. low-productivity systems, while herbivores exerted a stronger top-down effect in low-productivity systems. Effects of herbivores vs. nutrients also varied among algal functional groups (crustose algae, upright macroalgae, and filamentous algae), within a functional group between temperate and tropical systems, and according to the metric used to measure producer abundance. These analyses suggest that human alteration of food webs and nutrient availability have significant effects on primary producers but that the effects vary among latitudes and primary producers, and with the inherent productivity of ecosystems.     

Omnivory and grazer functional composition moderate cascading trophic effects in experimental <Emphasis Type="Italic">Fucus vesiculosus</Emphasis> habitats

We tested the relative strength of direct versus indirect effects of an aquatic omnivore depending on the functional composition of grazers by manipulating the presence of gastropod and amphipod grazers and omnivorous shrimp in outdoor mesocosms. By selectively preying upon amphipods and reducing their abundance by 70–80%, omnivorous shrimp favoured the dominance of gastropods. While gastropods were the main microalgal grazers, amphipods controlled macroalgal biomass in the experiment. However, strong predation on the amphipod by the shrimp had no significant indirect effects on macroalgal biomass, indicating that when amphipod abundances declined, complementary feeding by the omnivore on macroalgae may have suppressed a trophic cascade. Accordingly, in the absence of amphipods, the shrimp grazed significantly on green algae and thereby suppressed the diversity of the macroalgal community. Our experiment demonstrates direct consumer effects by an omnivore on both the grazer and producer trophic levels in an aquatic food web, regulated by prey availability.     

Distribution and morphological variation of low-shore algal turfs

The distribution of three functional groups of algae (filamentous, corticated terete and calcareous articulated) was investigated in low-shore, turf-forming assemblages from rocky shores in the western Mediterranean Sea (Italy). Algae were sampled along shores from three different stations. Shores were either exposed to wave action or sheltered and were characterised by various inclinations of the rocky substratum (horizontal, sloping and vertical). The hypotheses tested were: that the relative abundance of the three functional groups of algae (1) varies between sheltered and exposed shores, (2) is influenced by substratum inclination and (3) that these distributions are consistent across stations. There was no clear relationship between the morphology of turf-forming algae and shore characteristics. Covers of filamentous, corticated terete and calcareous articulated algae differed among shores and stations, but patterns were not consistent across shores with similar exposure to waves and substratum inclination. Turfs often comprised algae with different morphologies, in a combination of anchor and epiphytic species. It is suggested that functional groups of algae, as currently identified, are not effective in describing changes in distribution of algae between sheltered and exposed shores, when turf morphologies are considered. Future research on relationships between morphology and ecology of algae should consider the potential importance of positive interactions among associated species, which could override responses of individual functional groups.     

The role of the amphipod <Emphasis Type="Italic">Gammarus locusta</Emphasis> as a grazer on macroalgae in Swedish seagrass meadows

Mesograzers are thought to play a critical role in seagrass beds by preventing overgrowth of ephemeral algae. On the Swedish west coast, eelgrass Zostera marina has decreased in recent decades as a result of eutrophication and increased growth of macroalgal mats (mainly filamentous Ulva spp. and Ectocarpales), with no indication of grazer control of the algae. The aim of this study was to investigate the ability of the amphipod Gammarus locusta to control algal blooms during nutrient-enriched and ambient conditions, using a combination of laboratory, field and model studies. Laboratory experiments demonstrated that juvenile and adult G. locusta could consume both Ulva spp. and Ectocarpales, but that consumption of Ulva spp. was significantly higher. Cannibalism was common in individual treatments involving multiple size-classes of G. locusta, but only large, male gammarids consumed smaller juveniles in the presence of Ulva spp. as an alternative food source. However, no negative effects of cannibalism were found on total grazing impact. A model using size-specific grazing rates and growth rates of G. locusta and of Ulva spp. suggests that approximately 62 young juvenile, or 27 adult G. locusta are needed per gram DW of Ulva spp. to control the algal growth during ambient nutrient conditions, and approximately 2.6 times as many gammarids during enhanced nutrient conditions. On the Swedish west coast, densities and mean sizes of G. locusta in eelgrass beds are below these critical values, suggesting that the gammarids will not be able to control the growth of the filamentous macroalgae. However, in the field cage experiment, immigration of juveniles and reproduction of encaged adult G. locusta resulted in unexpectedly high densities of G. locusta (>4,000 individual m⁻²), and very low biomass of Ulva spp. in both ambient and nutrient-enriched treatments. Although the high numbers of juveniles in all cages precluded any significant treatment effects, this suggests that in the absent of predators, the population of G. locusta can grow significantly and control the biomass of Ulva spp. Furthermore, low grazing of Ectocarpales in the laboratory and high biomass of these filamentous brown algae in the field indicate a preference for the more palatable green algae Ulva spp. This study indicates that the high grazing capacity of G. locusta, in combination with high reproduction and growth rates, would allow the amphipod to play a key role in Z. marina ecosystems by controlling destructive blooms of filamentous green algae. However, high predation pressure appears to prevent large populations of G. locusta in eelgrass beds on the Swedish west coast today.     

Food preferences and related behavior of the browsing sea urchin <Emphasis Type="Italic">Tripneustes gratilla</Emphasis> (Linnaeus) and its potential for use as a biological control agent

The short-spined toxopneustid sea urchin Tripneustes gratilla feeds on a wide variety of algal species and on sea grasses. However, the urchin does show preferences when offered a selection of macroalgal species, which it encounters in nature. Preferences among macroalgae were evident in field-collected urchins exposed to pair-wise tests where the variable was either the consumption rate of the algae or observation of which algal species the urchins chose to touch with their lantern teeth. Exposure of lab-housed urchins to one of five species of macroalgae for 5 months did not seem to alter preferences of urchins in three of the exposure groups, but those exposed to Padina sanctae-crucis seemed to show an enhanced preference for this species when offered a choice of the five species of macroalgae at the end of the exposure period, and those exposed to Gracilaria salicornia seemed to avoid the species when offered the choice of the five species. Perhaps more ecologically important than their preferences were two other observations on these urchins: first, when offered only a single species of algae, the urchins on four of five diets ate the same quantity per day. Second, when simultaneously offered the choice among the five macroalgal species, the urchins consumed more macroalgae per day than when offered only one species. These urchins move about a meter a day. They probably encounter food resources in a relatively coarse-grained fashion and have evolved to eat what is available. Because of their limited movements, their habitat overlap with grazing fishes, their acceptance of a wide variety of macroalgae and their preference for macroalgae, these native urchins are thought to have the potential to serve as biological control agents of alien and invasive macroalgae, which have come to dominate some reef zones normally occupied by corals in Hawaii.     

Consumers affect prey biomass and diversity through resource partitioning

Consumer presence and nutrient availability can have contrasting and interactive effects on plant diversity. In a factorial experiment, we manipulated two levels of nutrient supply and the presence of two moderately specialized grazers in different combinations (no grazers, two species in monoculture, and both in combination). We tested how nutrients and grazers regulated the biomass of marine coastal epiphytes and the diversity of algal assemblages, based on the prediction that the effect of consumers on prey diversity depends on productivity and consumer specialization. Nutrient enrichment increased the epiphytic load, while monocultures of single grazer species partly prevented epiphyte growth. However, only the presence of two species with complementary feeding preferences effectively prevented epiphyte overgrowth. The epiphytes comprised micro- and macroalgal species, and the diversity of these algal assemblages differed, depending on grazer identity. For the microalgae, diversity was reduced by nutrient addition when grazer control was inefficient, but not when specialist microalgal grazers were present. Macroalgal diversity was reduced in ambient water with specialist macroalgal grazers compared to the treatment with inefficient ones. These results indicate that grazer composition and productivity are crucial in determining whether consumer pressure will have a positive or negative effect on algal diversity.     

Growth,nutrient uptake capacities and tissue constituents of the macroalgae Cladophora vagabunda and Gracilaria tikvahiae related to site-specific nitrogen loading rates

Cladophora vagabunda (L.) van den Hoek and Gracilaria tikvahiae (McLachlan) have become dominant components of the macroalgal assemblage in Waquoit Bay, a Massachusetts embayment, possibly due to nitrogen (N) enrichment from anthropogenic inputs transported via groundwater. During 1989–1993, we measured site-related growth, ammonium uptake rates and tissue constituents of these macroalgae from areas subject to high N loading rates (Childs River) and lower N loadings rates (Sage Lot Pond). We also conducted in situ and microcosm enrichment experiments to determine what limited algal growth throughout the year. Our results indicated that these species are strongly affected by and have a strong impact on the N environment of this embayment. For example, C. vagabunda and G. tikvahiae from Childs River had higher light-harvesting pigments and tissue-N concentrations than Sage Lot Pond populations. Additionally, both Childs River populations showed greater site-specific growth and N uptake rates, particularly during the summer period of peak growth. In fact, maximum uptake rates of 90 and over 140 mol dry wt g^-1 h^-1 for Childs River C. vagabunda and G. tikvahiae, respectively, suggest that these species can remove substanital quantities of N from overlying waters, and may be responsible for low (often (<1 M) water-column nutrient concentrations during summer. In situ and tank enrichment experiments indicated that growth rates were limited by available N during summer, while P may be limiting during a brief period toward the end of the annual growth cycle (autumn). Under experimental enrichment, growth rates of Sage Lot Pond algae were similar to values measured at the site receiving higher N inputs, and generally, G. tikvahiae showed growth enhancement (up to 0.2 doublings d^-1) under light-saturating conditions (0.5 m) while C. vagabunda showed nutrient-enhanced growth at 2.5 m. The effects of available nutrients on algal growth were strongly influenced by irradiance and temperature, resulting in a complex seasonal interaction that emphasized the dynamic nature of species response to N loading. Dominance by these two macroalgae in Waquoit Bay, as in other areas undergoing eutrophication, is likely related to physiological strategies that enable these species to tolerate large environmental variations, to take advantage of greater N availability and to survive indirect effects of N loading (e.g. reduced irradiance, anoxia).     

Larval settlement and metamorphosis of the mussel <Emphasis Type="Italic">Mytilus galloprovincialis</Emphasis> on different macroalgae

Settlement of mussels is commonly associated with macroalgae. The effects of 19 macroalgal species on the settlement and metamorphosis of pediveliger larvae of the mussel Mytilus galloprovincialis were investigated in the laboratory. Settlement and metamorphosis inducing activities of macroalgae Chlorodesmis fastigiata and Ceramium tenerrimum collected each month during the period between January 2005 and April 2006 were also investigated. Furthermore, C. fastigiata and C. tenerrimum were subjected to various treatments to investigate the roles of bacteria and diatoms on the algal surface in the induction of larval settlement and metamorphosis of M. galloprovincialis and the characteristics of the cues in these two macroalgae. Pediveliger larvae of M. galloprovincialis settled and metamorphosed in high percentages on Cladophora sp., Chlorodesmis fastigiata, Centroceras clavulatum, and Ceramium tenerrimum, all of which were filamentous in morphology. Macroalgae that were cylindrical, phylloid, flabellate, palmate and pinnate all showed low (<8%) percentages of post-larvae but four other filamentous macroalgae also had low mussel larval settlement, suggesting that chemical factors may also be involved. Seasonal variation had no effect on inductive activities of C. fastigiata and C. tenerrimum. Treatment of C. fastigiata and C. tenerrimum with formalin, ethanol and heat resulted in the significant decrease or loss of their inductive activities. Survival of algal cells within treated macroalgae also decreased significantly. Treatment of the two macroalgae with antibiotics and GeO₂ reduced the numbers of bacteria and diatoms on their surface but did not affect their inductive activities, indicating that the cue was produced by macroalgae and not by coexisting bacteria and diatoms. However, conditioned water and crude extracts of these two macroalgae did not induce larval settlement and metamorphosis. Thus, larvae of M. galloprovincialis settled and metamorphosed on specific filamentous macroalgae. The chemical cues produced by C. fastigiata and C. tenerrimum were susceptible to ethanol and heat treatments and were not recovered in the conditioned water nor in the extracts. The finding that inactive C. tenerrimum can be produced from culturing its apical segments provides a new tool to elucidate the chemical cue(s) from macroalgae through manipulation of their culture conditions.     

Chemoautotrophic symbiosis in the tropical clamSolemya occidentalis (Bivalvia: Protobranchia): ultrastructural and phylogenetic analysis

Patterns of algal seasonality, and their effect on the diet and feeding preferences of the herbivorous crab Grapsus albolineatus were investigated over an 18-mo period from March 1993 to August 1994 on an exposed tropical rocky shore (Hok Tsui Peninsula at Cape d' Aguilar, Hong Kong). Algal cover was greatest in the winter months, and lowest in the summer. Foliose algae such as Ulva fasciata, Porphyra suborbiculata, and Dermonema frappieri were dominant in the winter, but died off in the summer. During the hot summer months, perennial encrusting algae e.g., Ralfsia expansa, Hildenbrandia rubra, H. occidentalis, coralline crusts and the encrusting cyanobacteria Kyrtuthrix maculans, were the dominant algal species. Seasonal variation in algal abundance influenced the dietary selectivity of the herbivorous crab G. albolineatus. In the winter, the crab fed selectively on filamentous algae (e.g. Hincksia spp., Cladophora spp., Enteromorpha spp., and the cyanobacteria Lyngbya sp.). Foliose algae (e.g. U. fasciata, P. suborbiculata, Pterocladia tenuis) formed a small part of the diet, despite being the dominant species on the shore. Foliose and filamentous algae were virtually absent from the shore in the summer, and the crabs switched to feeding solely on encrusting algae. Electivity indices revealed preferences for green and brown turf species, and avoidance of foliose algae. Faecal analysis revealed that a greater proportion of the food is digested in the winter, suggesting that G. albolineatus is able to digest filamentous algae more efficiently than encrusting algae. Feeding preferences of G. albolineatus appear to be influenced by a number of factors, including the availability, digestibility and morphology of algae. The foraging behaviour and cheliped morphology of the crab also affect food choice. The monsoonal nature of Hong Kong's climate controls the diversity and abundance of intertidal algae and, therefore, indirectly influences the diet and subsequent growth and reproductive success of the herbivorous crab G. albolineatus.     

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.     

Emerson enhancement effect and quantum yield of photosynthesis for marine macroalgae in simulated underwater light fields

The contribution of enhancement to the total photosynthesis of marine macroalgae in their natural habitats was estimated by comparing the photosynthesis measured by O₂-electrode in five broad-band light fields with that predicted (on the assumption that no enhancement was occurring) from the photosynthetic action spectrum of each plant and the spectral distribution of the light fields. The excess of measured values divided by calculated values provided a measure of enhancement. Although 37% enhancement was observed for red algae in unfiltered quartz-iodine light, and 18% for green and brown algae, substantially lower values were obtained for all species in more natural light fields. In those typical of shallow coastal waters, phycoerythrin-rich red algae exhibited 15 to 20% enhancement, but little enhancement (<5%) was detected in other algae. In a green light field, representing deep coastal water, there was no significant enhancement in any species, and only green and brown algae showed any enhancement (ca 8%) in broad-band blue light, similar to that in deep oceanic waters. Quantum yields of 0.09 to 0.10 O₂ molecules per absorbed photon were recorded in most light fields for green and brown algae with thin thalli, but yields decreased in the blue light field and in species with thicker thalli. All red algae had quantum yields of about 0.08 O₂ molecules per absorbed photon, except in the blue light field, in which quantum yields were reduced by 70%.     

UV-radiation can affect depth-zonation of Antarctic macroalgae

Due to depletion of stratospheric ozone over polar regions of the Northern and Southern Hemispheres UV-B-radiation has increased at the surface of the earth. Measurements of variable chlorophyll fluorescence were conducted to document UV-induced photoinhibition of photosystem II in cultivated macroalgae with different depth distributions in Antarctica. The reactions during artificial UV-exposure were observed on a short time scale (hours) and in light–dark cycles over several days. The nine species of investigated macroalgae show great differences in UV-tolerance of the photosynthetic process. Photosynthesis of the studied green algae was inhibited to a minor degree, while the brown algae showed an intermediate inhibition of photosynthesis. The response of the studied red algae varied with species. The differences in the degree of inhibition and recovery of photosynthetic efficiency and capacity indicate that UV-radiation is one important factor affecting the vertical distribution of macroalgae in nature. Received: 11 December 1997 / Accepted: 27 March 1998     

绿潮藻类暴发对天鹅湖水体和沉积物磷含量的影响

在荣成天鹅湖藻类暴发区域采集新鲜沉积物和丝状硬毛藻（Chaetomorpha spp.）,进行室内模拟试验,监测了生长过程中硬毛藻的生物量、磷富集量以及不同处理水体总磷（TP）和可溶性磷（SRP）质量浓度的变化,并分析了藻类生长对沉积物中各形态磷含量的影响。结果表明,当水体磷含量较高时,硬毛藻生长较快,相对生长速率高达14.88%,之后随着水体磷浓度的下降,生长速率逐渐减小。不同处理间硬毛藻的生物量相差很大,高磷含量处理显著高于低磷处理,最大差值可达26.50 g。随着藻类的生长,水体TP和SRP含量明显降低,其中高磷含量处理的TP质量浓度由0.93 mg·L-1降至0.01 mg·L-1,低磷含量处理水体SRP质量浓度均降至0.006 mg·L-1以下。当水体磷含量降至一定水平,沉积物中磷可向水体释放,其中可还原态磷和铁铝结合态磷的降幅分别为23.98%和12.61%。在高磷含量处理组,藻体中磷的富集量显著升高,且当水体磷含量相同的条件下,有沉积物处理的富集量显著高于无沉积物处理。相关分析表明,藻体生物量与水体TP和SRP的相关性较好,其中高磷含量处理组生物量与水体TP、SRP呈高度负相关,而相对生长速率与之呈显著正相关。结果说明,水体及沉积物中磷均可作为硬毛藻生长的营养来源;另一方面,藻类生长可明显降低水体磷含量,并促进沉积物中磷的释放。