Nectar provision attracts hummingbirds and connects interaction networks across habitats

Many ecosystems have been modified by humans, creating novel habitats that include human-provided resources. Gardens adjacent to native habitats may affect plant–pollinator interactions by altering the determinants of interactions and species specialization. Here, we characterized a network comprising plants and hummingbirds interacting in a birdwatching garden with human-provided resources (nectar feeders and exotic plants) and adjacent Andean cloud forest in Colombia. Specifically, we investigated the proportion of hummingbirds visiting feeders and native/exotic plants to evaluate the connection between the habitats and the ecological determinants of the interaction network. Hummingbirds relied heavily on artificial nectar feeders in the garden, leaving the natural cloud forest for resources. Morphological matching was the single most important predictor of the observed pairwise interactions, for both hummingbirds and plants. At the species level, longer flowering phenology and a higher amount of sugar in nectar led to a higher degree for plants (i.e. the number of visiting hummingbird species). In contrast, a longer floral corolla was associated with lower specialization. Abundance was the best predictor of the number of partners for hummingbirds. The garden created for birdwatching attracted most, but not all, hummingbird species beyond their natural cloud forest habitat. Interestingly, the most frequently visited plants in the garden were native, especially the endemic and endangered tree Zygia lehmannii (Fabaceae). Our results show that some ecological mechanisms determining interactions in natural communities still hold in intensively modified habitats. Furthermore, a compromise between conservation and hummingbirds’ attraction to birding lodges/gardens is possible, for instance by favouring native and endemic plant species that are highly attractive for pollinators.     

Resource limitation and fish predation: their importance to mobile epifauna associated with JapaneseSargassum

The possibility that resource limits constrain the growth of mobile epifaunal populations associated withSargassum patens plants was investigated by placing plants and associated animals into field microcosms which excluded fish predators, and then comparing faunal abundance and size-structure changes in different microcosm treatments with field populations. Four different micrososm treatments were set up: two treatments containing defaunated plants inoculated with caprellid amphipods, and two control treatments with natural faunas. The estimated secondary production of faunas enclosed in all microcosm treatments rapidly settled on a constant value (5 mg/day) which was similar to that determined in experiments conducted in Western Australia using the same microcosms but for faunas associated with a seagrass rather than a macroalga. These results support the hypothesis that the secondary production of epifaunal communities associated with macrophytes is constrained by quantifiable food resource ceilings. Predation by the most common fish species in the area, the wrasseHalichoeres tenuispinis, did not appear to alter macrofaunal production in theS. patens bed; however, it did greatly affect the faunal size-structure by eliminating most of the larger animals. The majority of epifaunal animals 2.0 mm sieve-size were consumed byH. tenuispinis, while negligible numbers of 0.5-mm sieve-size animals were captured. We postulate that food resource ceilings and predatory size-selectivity are widespread phenomena, affecting epifaunal populations at a variety of locations. Predation is predicted to generally increase rather than decrease faunal abundance because the consumption of each large invertebrate by a predator frees sufficient resources to feed several smaller individuals.     

Migration from plant to plant: an important factor controlling densities of the epiphytic cladoceran <Emphasis Type="Italic">Alona</Emphasis> (Chydoridae,Anomopoda) on lake vegetation

We investigated seasonal changes in the density of epiphytic cladocerans Alona spp. (Chydoridae, Anomopoda) in two habitats, emergent and submerged aquatic plants, in Lake Suwa, Japan, from April to August 1998 and from April to November 2000. Alona had a density peak in early June on reeds (emergent) and in late June on Potamogeton malaianus (submerged). In summer, Alona density remained low in both habitats. Although density was positively correlated with the abundance of epiphytic algae, the birth rate was constant and no correlation between algal abundance and clutch size was detected. In a field experiment using ropes as an artificial substrate covered with high and low densities of epiphytic algae as food, more Alona attached to the ropes with the high density of algae. These results suggest that Alona may select food-rich habitats and migrate seasonally, and that migration is an important factor in the population dynamics of epiphytic chydorid cladocerans such as Alona. In Lake Suwa, Alona may migrate from the reed zone to the submerged macrophyte zone in June.     

Influences of habitat complexity on the diversity and abundance of epiphytic invertebrates on plants

SUMMARY 1. The compound influence of habitat complexity and patch size on stream invertebrate assemblages associated with submerged macrophytes was investigated through field sampling of two natural macrophyte species with contrasting leaf morphologies (complex, Ranunculus yezoensis; simple, Sparganium emersum) and an experiment with two artificial plants with different levels of morphological complexity. 2. The artificial plant experiment was designed to separate the effects of habitat area (patch size) and habitat complexity, thus enabling a more rigorous assessment of complexity per se than in previous studies where only a single patch size was used. Simple and complex artificial plants were established with five different patch sizes corresponding to the range found in natural plants. 3. Invertebrates occurred on both complex and simple forms of natural and artificial plants at similar abundances with dipterans and ephemeropterans being predominant. Taxon richness was higher on structurally complex Ranunculus than on simple Sparganium and was similarly higher on the complex artificial plant than on the simple one, over the entire range of habitat patch sizes. Thus, architectural complexity affected the taxon richness of epiphytic invertebrates, independently of habitat scale. 4. On the natural plants there was no difference in the abundance (both number of individuals and biomass) of invertebrates between simple and complex forms, while on artificial plants more invertebrates occurred on complex than on simple forms. The amount of particulate organic matter, >225 μm (POM) and chlorophyll a showed mixed patterns on natural and artificial plants, suggesting that the availability of these resources is not an overriding proximate factor controlling invertebrate abundance on plants. The difficulty of extrapolating from experimental results involving use of artificial plants is discussed, especially when considering the relationship between habitat structure and the occurrence of epiphytic invertebrates on natural plants.     

Particle size selection in individuals from epifaunal versus infaunal populations of the nereidid polychaete Neanthes succinea(Polychaeta: Nereididae)

Neanthes succinea (Frey & Leuckart, 1847) is a common nereidid polychaete of both epifaunal and infaunal estuarine habitats. The gut contents of individuals collected from two epifaunal and two infaunal habitats are compared. Our a priori expectation was that individuals from epifaunal habitats would be classified as macrophagous with guts indicating carnivory and/or macroalgal herbivory, while individuals from infaunal habitats would be classified as microphagous with guts indicating deposit feeding. At all four locations gut contents indicated deposit feeding with little indication of macrophagous feeding. Average particle sizes for mineral grains did not differ between the four collection sites. For the two infaunal locations mean size of the mineral grains in gut contents was significantly smaller than ambient sediments. In addition to mineral grains, guts contained diatoms, dinoflagellates, macrophytic detritus, protozoan tests, and a variety of metazoans. Our study demonstrates that caution is necessary when inferring feeding type from morphology and that population and habitat specific differences in diet can occur within the same species.     

Biomass allocation patterns in terrestrial, epiphytic and aquatic species of Utricularia (Lentibulariaceae)

Utricularia forms the largest genus of carnivorous plants and is characterized by the possession of typical traps (“bladders”). Total biomass allocation was examined in three aquatic, six terrestrial and one epiphytic species of Utricularia from natural habitats in West Africa and from the Botanical Gardens, Bonn. Total biomass of aquatic species was considerably higher than that of terrestrial or epiphytic species. Epiphytic Utricularia accumulate about 35% of their biomass in green leaves, in contrast to 65% of nearly chlorophylless reproductive structures and traps. Aquatic species allocated more than 85% of their total biomass to stolons, leaves and traps, but only 10–13% to reproductive structures. This is in stark contrast to the allocation patterns of terrestrial bladderworts. These species allocate nearly 90% of their total biomass in reproductive structures, and only about 10% to stolons, leaves and traps. This reduction of photosynthetically active plant tissue strongly suggests that as a consequence of the alternative resource of chemical energy, the carnivorous habit might have partly replaced autotrophy in certain terrestrial Utricularia species, especially in some smaller ones.     

Nutrient impacts on epifaunal density and species composition in a subtropical seagrass bed

The capacity of epifauna to control algal proliferation following nutrient input depends on responses of both grazers and upper trophic level consumers to enrichment. We examined the responses of Thalassia testudinum (turtle grass) epifaunal assemblages to nutrient enrichment at two sites in Florida Bay with varying levels of phosphorus limitation. We compared epifaunal density, biomass, and species diversity in 2 m² plots that had either ambient nutrient concentrations or had been enriched with nitrogen and phosphorus for 6 months. At the severely P-limited site, total epifaunal density and biomass were two times higher in enriched than in unenriched plots. Caridean shrimp, grazing isopods, and gammarid amphipods accounted for much of the increase in density; brachyuran crabs, primary predatory fish, and detritivorous sea cucumbers accounted for most of the increase in biomass. At the less P-limited site, total epifaunal density and biomass were not affected by nutrient addition, although there were more caridean shrimp and higher brachyuran crab and pink shrimp biomass in enriched plots. At both sites, some variation in epifaunal density and biomass was explained by features of the macrophyte canopy, such as T. testudinum and Halodule wrightii percent cover, suggesting that enrichment may change the refuge value of the macrophyte canopy for epifauna. Additional variation in epifaunal density and biomass was explained by epiphyte pigment concentrations, suggesting that enrichment may change the microalgal food resources that support grazing epifauna. Increased epifaunal density in enriched plots suggests that grazers may be able to control epiphytic algal proliferation following moderate nutrient input to Florida Bay. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Tests of artificial substrata as nursery habitat for young fish

Recruitment areas for freshwater fish are often negatively affected by eutrophication and physical disturbances. Vegetated areas, which are important nursery habitats, are reduced and water turbidity increased. As a method of compensation, we tested artificial substrata for young-of-the-year fish. The structures were made of spruce bundles, with and without surrounding nets, and placed in a hyper-eutrophic very turbid environment and in an undisturbed area with clear water. Both habitats were devoid of submerged vegetation. Young fish abundance in treated areas was compared with adjacent reference sites. In the clear water area, the abundance of all investigated species – perch, pike, bream, silver-bream and roach – was higher in areas with artificial refuges. A similar response was evident for cyprinids in the turbid environment. High abundance of pikeperch and ruffe appeared in the hyper-eutrophic test area. Neither of these species, nor perch, was attracted to the artificial refuges. The lack of response in perch and pikeperch suggests that the importance of structural refuge decreases in very turbid water for these species. Of the two methods tested, spruce bundles with surrounding nets generally attracted most young fish, implying that the nets further increased the refuge capacity by reducing predation risk. The conclusion is that artificial habitats could improve recruitment habitats and that protective devices can increase refuge capacity.     

晋西北丘陵风沙区柠条林下草本植物群落组成和种群生态位变化特征

晋西北丘陵风沙区生态环境脆弱,是我国风沙活动危害最为严重的地区之一。为探究晋西北丘陵风沙区人工柠条林林下草本植物群落组成和种群生态位变化规律,以不同种植年限(撂荒地CK、6、12、18、40、50 a)人工柠条林林下天然草本植物群落作为研究对象,对其群落组成、重要值及种群生态位变化特征进行分析。研究结果表明：(1)不同年限柠条林下天然草本植物组成共记录到22科41属52种,其中,种植前期(0—6 a)林下草本植物以一年生草本植物为优势类群,伴有少数多年生草本植物(3种);种植中期(12—18 a)林下草本植物主要由多年生草本植物(12种)为主,伴生有一年生草本植物(6种)及天然灌木(2种)组成;在种植后期(40—50 a),林下草本植物依然以多年生草本植物(12种)为主。(2)在不同年限柠条林下,草本植物生态位宽度和生态位重叠发生明显变化,种植前期(0—12 a)的优势类群为一年生先锋物种(米蒿和野燕麦),其生态位宽度最宽(9.46、9.34),且与其他物种的生态位重叠程度最大(0.3、0.29);而种植中后期(18—50 a)优势类群变为多年生草本植物,优势种披碱草与其他物种的生态位...     

Early faunal successional patterns in artificial reefs used for restoration of impacted biogenic habitats

This study describes the early epifaunal succession associated with an artificial reef constructed to regenerate damaged biogenic habitats formed by Modiolus modiolus (Linnaeus, 1758). Clumps of live M. modiolus were translocated onto three treatments: flattened cultch, elevated cultch, and directly onto the sea floor. Photographic surveys were carried out 1, 6, and 12 months after completion of the experimental array to test the hypothesis that the artificial reef would enhance habitat complexity thus increasing biodiversity and accelerating faunal community succession. These effects were predicted to be greater on elevated cultch due to higher level of protection and greater accessibility to food compared to sea floor treatments. Univariate analysis indicated that after 12 months the artificial reef had developed a significantly richer and more diverse community compared to 1- and 6-month stages. Multivariate analysis revealed a significant temporal shift in species composition from mobile taxa to sessile and interstitial macroinvertebrates as the artificial reef settled. Reef elevation offered no significant advantages for the development of the epifaunal assemblage. Although further regular monitoring is advisable, this study demonstrated that translocation of a foundation species can help restore marine benthic habitats through the development of a diverse community in a relatively short time.     

Marine dock pilings foster diverse,native cryptobenthic fish assemblages across bioregions

Anthropogenic habitats are increasingly prevalent in coastal marine environments. Previous research on sessile epifauna suggests that artificial habitats act as a refuge for nonindigenous species, which results in highly homogenous communities across locations. However, vertebrate assemblages that live in association with artificial habitats are poorly understood. Here, we quantify the biodiversity of small, cryptic (henceforth “cryptobenthic”) fishes from marine dock pilings across six locations over 35° of latitude from Maine to Panama. We also compare assemblages from dock pilings to natural habitats in the two southernmost locations (Panama and Belize). Our results suggest that the biodiversity patterns of cryptobenthic fishes from dock pilings follow a Latitudinal Diversity Gradient (LDG), with average local and regional diversity declining sharply with increasing latitude. Furthermore, a strong correlation between community composition and spatial distance suggests distinct regional assemblages of cryptobenthic fishes. Cryptobenthic fish assemblages from dock pilings in Belize and Panama were less diverse and had lower densities than nearby reef habitats. However, dock pilings harbored almost exclusively native species, including two species of conservation concern absent from nearby natural habitats. Our results suggest that, in contrast to sessile epifaunal assemblages on artificial substrates, artificial marine habitats can harbor diverse, regionally characteristic assemblages of vertebrates that follow macroecological patterns that are well documented for natural habitats. We therefore posit that, although dock pilings cannot function as a replacement for natural habitats, dock pilings may provide cost‐effective means to preserve native vertebrate biodiversity, and provide a habitat that can be relatively easily monitored to track the status and trends of fish biodiversity in highly urbanized coastal marine environments.     

The relative importance of food and shelter for seagrass-associated invertebrates: a latitudinal comparison of habitat choice by isopod grazers

The generality of mechanisms affecting habitat choice and grazing in seagrass meadows was evaluated in a latitudinal comparison of seagrass grazers from the temperate (60°N) Baltic Sea and the subtropical (30°N) Gulf of Mexico. Using similar habitat choice experiment set-ups in Finland and the USA, the role of food type, habitat complexity and predation hazard on habitat choice of the isopods Idotea baltica (Pallas) and Erichsonella attenuata Harger were tested. When shelter was provided by both living and artificial seagrass, epiphytic food resources on artificial vegetation were clearly preferred by both species, although Idotea was attracted to epiphyte-free seagrass when no alternative food was present. When choosing between food and shelter, both species preferred epiphytic food over shelter. However, under predation hazard of fish, Erichsonella clearly switched to the habitat offering shelter, while the presence of a predatory fish produced no preference for shelter by Idotea. Food type may be considered as an universal mechanism that partly determines the presence of grazers in seagrass habitats and is, in the absence of a predator, more important than shelter. Predation risk affected the behaviour of the grazers, but the response varied between species possibly due to varying importance of fish predation in the areas studied. Received: 16 November 1998 / Accepted: 13 February 1999     

Effect of environmental factors on the spatial distribution of the epifauna of the alga Halopteris scoparia in Algeciras Bay, Southern Spain

The physical characteristics and environmental versatility of the alga Halopteris scoparia (Phaeophyta, Sphacelariales) make it a suitable substrate for development of epiphytic communities. Spatial variation of the epifauna on this alga in Algeciras Bay (southern Spain) in response to different environmental conditions is investigated. There is a clear difference in community composition between external and internal areas of the bay, with an important group of species present in only one of the areas (e.g., in outer areas crustaceans such as Tanais dulongii or Amphilochus neapolitanus or the polychaete Nicolea venustula; and species from inner areas such as the crustacean Jassa marmorata and the mollusc Alvania montagui or Rissoa similis). The external zone shows high hydrodynamics and low sedimentation rates, whereas in the internal one, there is a high sedimentation rate (as a result of two main rivers, a less strong current regime, and the presence of urban and industrial wastes). The conditions prevailing in the internal zone of the bay are unfavourable for most of the epifaunal species in the external bay areas.     

Artificial substrata in a shallow sublittoral habitat: do they adequately represent natural habitats or the local species pool?

Artificial substrata have been advocated as tools which have considerable potential for monitoring both natural and anthropogenic effects on invertebrate communities of shallow coastal environments. In this experiment, community structure was compared between two dominant natural algal habitats (kelp holdfasts and algal turf) and artificial substratum units (ASUs; nests of pan scourers) deployed in close contact with, and 20 cm above the substratum. Univariate and multivariate statistical analyses were applied to the data to determine the similarity of community structure between the four different habitats. In addition, recently developed measures of taxonomic distinctness were applied to the data from both sets of artificial substrata to determine if they provided a representative sample of the local epifaunal species pool and thus have the potential to be used as surrogate samples for this important faunal group. There were marked differences between community structure in each of the habitats. Both sets of artificial substrata were dominated by tubicolous polychaetes with abundances that were more than an order of magnitude greater than in the holdfast and turf samples. The fauna recruiting to the artificial substrata deployed above the substratum showed the lowest values in the univariate summaries of diversity and evenness and were unrepresentative of the local species pool. Artificial samples deployed in contact with the substratum showed greater diversity and evenness but were still mostly unrepresentative of the local species pool. The tendency for both sets of artificial substrata to under-sample amphipods and to be dominated by suspension-feeding polychaetes suggests that methods using these units may be relatively insensitive to the effects of anthropogenic impacts (e.g. sewage outfalls) where shifts in community structure including increased dominance of suspension-feeders and polychaetes and a reduced dominance of amphipods have been observed. Further studies, including the evaluation of temporal variation in community structure related to the time at which the ASUs are deployed and duration of deployment, are needed to test the wider utility of artificial substrata as tools for monitoring shallow, sublittoral, epifaunal communities.     

Patterns of abundance and assemblage structure of epifauna inhabiting two morphologically different kelp holdfasts

The mobile fauna associated with two sympatric kelp species with different holdfast morphology (Saccorhiza polyschides and Laminaria hyperborea) was compared to test for differences in the assemblage structure of holdfast-associated mobile epifauna. A total of 24,140 epifaunal individuals were counted from 30 holdfasts of each kelp species. Overall epifaunal abundances exceeded faunal abundances previously reported from holdfasts of other kelps. Three taxonomic groups, Amphipoda, Mollusca, and Polychaeta, accounted for ca. 85% of all individuals. Total abundances increased with the amount of habitat available, quantified either as the volume or the area provided by the holdfasts. The multivariate structure of the epifaunal assemblage did not differ between holdfasts of the two kelp species. However, epifaunal assemblages responded differentially to the habitat attributes provided by each type of kelp holdfast: multivariate variation in the assemblage structure of epifauna was mostly explained by holdfast area and volume for L. hyperborea, and by the surface-to-volume ratio for S. polyschides holdfasts. Therefore, the physical attributes of biogenic habitats, here kelp holdfasts that better predict patterns in the assemblage structure of associated fauna can differ according to their different physical morphology, even though the overall assemblage structure of associated fauna was similar.     

Influence of Habitat and Climate Variables on Arbuscular Mycorrhizal Fungus Community Distribution,as Revealed by a Case Study of Facultative Plant Epiphytism under Semiarid Conditions

In semiarid Mediterranean ecosystems, epiphytic plant species are practically absent, and only some species of palm trees can support epiphytes growing in their lower crown area, such as Phoenix dactylifera L. (date palm). In this study, we focused on Sonchus tenerrimus L. plants growing as facultative epiphytes in P. dactylifera and its terrestrial forms growing in adjacent soils. Our aim was to determine the possible presence of arbuscular mycorrhizal fungi (AMF) in these peculiar habitats and to relate AMF communities with climatic variations. We investigated the AMF community composition of epiphytic and terrestrial S. tenerrimus plants along a temperature and precipitation gradient across 12 localities. Epiphytic roots were colonized by AMF, as determined by microscopic observation; all of the epiphytic and terrestrial samples analyzed showed AMF sequences from taxa belonging to the phylum Glomeromycota, which were grouped in 30 AMF operational taxonomic units. The AMF community composition was clearly different between epiphytic and terrestrial root samples, and this could be attributable to dispersal constraints and/or the contrasting environmental and ecophysiological conditions prevailing in each habitat. Across sites, the richness and diversity of terrestrial AMF communities was positively correlated with rainfall amount during the most recent growing season. In contrast, there was no significant correlation between climate variables and AMF richness and diversity for epiphytic AMF communities, which suggests that the composition of AMF communities in epiphytic habitats appears to be largely determined by the availability and dispersion of fungal propagules from adjacent terrestrial habitats.     

Boxing for biodiversity: evaluation of an artificially created decaying wood habitat

Many saproxylic species are threatened in Europe because of habitat decline. Hollow trees represent an important habitat for saproxylic species. Artificial habitats may need to be created to maintain or increase the amount of habitat due to natural habitat decline. This study investigated the extent to which saproxylic beetles use artificial habitats in wooden boxes. The boxes were placed at various distances (0–1800 m) from known biodiversity hotspots with hollow oaks and studied over 10 years. Boxes were mainly filled with oak saw dust, oak leaves, hay and lucerne flour. In total, 2170 specimens of 91 saproxylic beetle species were sampled in 43 boxes. The abundance of species associated with tree hollows, wood rot and animal nests increased from the fourth to the final year, but species richness declined for all groups. This study shows that wooden boxes can function as saproxylic species habitats. The artificial habitats developed into a more hollow-like environment during the decade long experiment with fewer but more abundant tree hollow specialists.     

Effects of eutrophication and snails on Eurasian watermilfoil (Myriophyllum spicatum) invasion

Exotic species can invade and establish new habitats both as a result of their own traits, and as a result of the characteristics of the environment they invade. Here, we show that the abundance of the invasive submerged aquatic plant, Myriophyllum spicatum (Eurasian watermilfoil) is highly dependent on the conditions of the environment in a mesocosm experiment. M. spicatum is allelopathic towards epiphytic algae, and in the absence of algivorous snails, we found that the abundance of both algae and M. spicatum significantly increased with experimentally increased nutrient loading, while the abundance of native submerged macrophytes declined. However, when snails were present, snail biomass increased with increasing nutrient loading, and M. spicatum biomass was consistently low while native submerged macrophyte biomass was consistently high. Our results stress the importance of the interaction between species traits and environmental conditions when considering the invasiveness of certain exotic species and the invasibility of certain environments.     

Ecological aspects of syllids (Annelida: Polychaeta: Syllidae) on Thalassia testudinum beds in Venezuela

The spatial and temporal variations of syllids associated with the tropical Thalassia testudinum were studied at four seagrass beds in the Morrocoy National Park. The epifaunal syllids were collected at quarterly intervals throughout one year using a modified suction sampler. Possible relationships between sampling sites, months and species were evaluated by Principal Components Analysis. The spatial-temporal variability of the predominant species was tested by means of a 2-factor ANOVA with the sampling sites and months as factors. A total of 1138 individual syllids were collected, belonging to 41 species and 12 genera, of which Branchiosyllis, Exogone, Odontosyllis, Sphaerosyllis and Syllis showed the highest diversity. Spatial variations were defined by the species of Branchiosyllis (B. exilis, B. lorenae, B. oculata) whereas temporal variability was defined by certain species of Syllis(S. beneliahui, S. broomensis and S. prolifera), the latter also being the most species rich genus present. Of the 41 species found, only these six showed consistent presence–absence and abundance patterns. Species richness and abundance were significantly higher in March at all sampling sites. Collectively, the syllid fauna of the Morrocoy seagrass beds was richer than recorded from other similar habitats. These findings are discussed in relation to the physical and biotic factors that may affect the variability of syllid populations in these seagrass habitats.     

Influence of light and nitrogen on the phlorotannin content of the brown seaweeds Ascophyllum nodosum and Fucus vesiculosus

Phlorotannins, C-based defence compounds in brown seaweeds, show a high degree of spatial and temporal variation within seaweed species. One important model explaining this variation is the Carbon Nutrient Balance Model (CNBM), which states that the relative supply of carbon and limiting nutrients will determine the level of defence compounds in plants. Nitrogen is often considered to be the limiting nutrient for marine macroalgal growth and the CNBM thus predicts that when the carbon:nitrogen ratio is high, photosynthetically fixed carbon will be allocated to production of phlorotannins. In the present study, we evaluated the effects of light (i.e. carbon) and nitrogen on the phlorotannin content of two intertidal brown seaweeds, Ascophyllum nodosum and Fucus vesiculosus. This was done in an observational field study, as well as in a manipulative experiment where plants from habitats with different light regimes were subjected to different nitrogen and light treatments, and their phlorotannin content was measured after 14 days. The results showed that there was a negative relationship between tissue nitrogen and phlorotannin content in natural populations of F. vesiculosus, but not in A. nodosum. In the short term, the phlorotannin content in both algal species was not affected by changes in nitrogen availability. Exposure to sunlight had a positive effect on the phlorotannin content in natural populations of both algal species but, in the manipulative experiment, only F. vesiculosus showed a rapid response to changes in light intensities. Plants subjected to sunlight contained higher phlorotannin content than shaded plants. In conclusion, the results imply that nitrogen availability explains some of the natural variation in the phlorotannin content of F. vesiculosus, but the light environment has greater importance than nitrogen availability in predicting the phlorotannin content of each species.