INVERTEBRATE‐MEDIATED NUTRIENT LOADING INCREASES GROWTH OF AN INTERTIDAL MACROALGA1

Even in nitrogen‐replete ecosystems, microhabitats exist where local‐scale nutrient limitation occurs. For example, coastal waters of the northeastern Pacific Ocean are characterized by high nitrate concentrations associated with upwelling. However, macroalgae living in high‐zone tide pools on adjacent rocky shores are isolated from this upwelled nitrate for extended periods of time, leading to nutrient limitation. When high‐intertidal pools are isolated during low tide, invertebrate‐excreted ammonium accumulates, providing a potential nitrogen source for macroalgae. I quantified the influence of mussels (Mytilus californianus Conrad) on ammonium accumulation rates in tide pools. I then evaluated the effects of ammonium loading by mussels on nitrogen assimilation and growth rates of Odonthalia floccosa (Esp.) Falkenb., a common red algal inhabitant of pools on northeastern Pacific rocky shores. Odonthalia was grown in artificial tide pool mesocosms in the presence and absence of mussels. Mesocosms were subjected to a simulated tidal cycle mimicking emersion and immersion patterns of high‐intertidal pools on the central Oregon coast. In the presence of mussels, ammonium accumulated more quickly in the mesocosms, resulting in increased rates of nitrogen assimilation into algal tissues. These increased nitrogen assimilation rates were primarily associated with higher growth rates. In mesocosms containing mussels, Odonthalia individuals added 41% more biomass than in mesocosms without mussels. This direct positive effect of mussels on macroalgal biomass represents an often overlooked interaction between macroalgae and invertebrates. In nutrient‐limited microhabitats, such as high‐intertidal pools, invertebrate‐excreted ammonium is likely an important local‐scale contributor to macroalgal productivity.     

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.     

Top-down and bottom-up factors in tidepool communities

Recent studies suggest that nutrient variation influences rocky intertidal community structure, however empirical evidence is rare. In the Gulf of Maine, tidepools that occur on seagull feeding roosts are potentially subjected to regular nutrient loading from seagull guano. The results of a survey conducted on Swan's Island, ME show that roost tidepools have very low macroinvertebrate and macroalgal diversity as well as very high phytoplankton biomass compared to non-roost tidepools. An experiment presented here tested basic food chain hypotheses in tidepool communities. These basic food chain models predict that in a tidepool with one trophic level (phytoplankton only), phytoplankton biomass will increase when nutrients are enriched. In contrast, these models predict that in two trophic level tidepools (phytoplankton and mussels) herbivory will prevent an increase in phytoplankton biomass when nutrients are enriched. A short term 2×2 factorially designed field experiment was used to test this basic conceptual model using herbivory by mussels and enrichment with nitrogen as the main effects. The results of this investigation are consistent with the predictions of basic food chain models, and indicate that over the short time interval of a few days, herbivory by mussels is sufficient to maintain low phytoplankton levels following enrichment with nitrogen. Experimental enrichment with phosphorus in this study had no effect on phytoplankton biomass. The results of this study suggest that periodic pulses of nitrogen into tidepools will have little effect on phytoplankton biomass when mussels are present and that longer-term chronic nitrogen influxes may be driving the patterns of community structure in tidepools occurring on roosts.     

Nitrate uptake varies with tide height and nutrient availability in the intertidal seaweed Fucus vesiculosus

Intertidal seaweeds must cope with a suite of stressors imposed by aerial exposure at low tide, including nutrient limitation due to emersion. Seaweeds can access nutrients only when submerged, so individuals living higher compared to lower on the shore may have adaptations allowing them to acquire sufficient amounts of nutrients to survive and maintain growth. Using a combination of observations and experiments, we aimed to identify intraspecific variation in nitrate uptake rates across the intertidal distribution of F. vesiculosus, as well as test for acclimation in response to a change in tide height. We replicated our study at sites spanning nearly the entire Gulf of Maine coastline, to examine how local environmental variability may alter intraspecific variation in nitrate uptake. We found that average nitrate uptake rates were ~18% higher in upper compared to lower intertidal Fucus vesiculosus. Furthermore, we found evidence for both acclimation and adaptation to tide height during a transplant experiment. F. vesiculosus transplanted from the lower to the upper intertidal zone was characterized by increased nitrate uptake, but individuals transplanted from the upper to the lower intertidal zone retained high uptake rates. Our observations differed among Gulf of Maine regions and among time points of our study. Importantly, these differences may reflect associations between nitrate uptake rates and abiotic environmental conditions and seaweed nutrient status. Our study highlights the importance of long‐term variation in ambient nutrient supply in driving intraspecific variation of seaweeds across the intertidal gradient and local and seasonal variation in ambient nutrient levels in mediating intraspecific differences.     

Causes and consequences of thermal tolerance limits in rocky intertidal porcelain crabs, genus petrolisthes

Vertical zonation of intertidal organisms, from the shallowsubtidal to the supralittoral zones, is a ubiquitous featureof temperate and tropical rocky shores. Organisms that livehigher on the shore experience larger daily and seasonal fluctuationsin microhabitat conditions, due to their greater exposure toterrestrial conditions during emersion. Comparative analysesof the adaptive linkage between physiological tolerance limitsand vertical distribution are the most powerful when the studyspecies are closely related and occur in discrete vertical zonesthroughout the intertidal range. Here, I summarize work on thephysiological tolerance limits of rocky intertidal zone porcelaincrab species of the genus Petrolisthes to emersion-related heatstress. In the eastern Pacific, Petrolisthes species live throughouttemperate and tropical regions, and are found in discrete verticalintertidal zones in each region. Whole organism thermal tolerancelimits of Petrolisthes species, and thermal limits of heartand nerve function reflect microhabitat conditions. Speciesliving higher in the intertidal zone are more eurythermal thanlow-intertidal congeners, tropical species have the highestthermal limits, and the differences in thermal tolerance betweenlow- and high-intertidal species is greatest for temperate crabs.Acclimation of thermal limits of high-intertidal species isrestricted as compared to low-intertidal species. Thus, becausethermal limits of high-intertidal species are near current habitattemperature maxima, global warming could most strongly impactintertidal species.     

Seaweed biogeochemistry: Global assessment of C:N and C:P ratios and implications for ocean afforestation

Algal carbon-to-nitrogen (C:N) and carbon-to-phosphorus (C:P) ratios are fundamental for understanding many oceanic biogeochemical processes, such as nutrient flux and climate regulation. We synthesized literature data (444 species, >400 locations) and collected original samples from Tasmania, Australia (51 species, 10 locations) to update the global ratios of seaweed carbon-to-nitrogen (C:N) and carbon-to-phosphorus (C:P). The updated global mean molar ratio for seaweed C:N is 20 (ranging from 6 to 123) and for C:P is 801 (ranging from 76 to 4102). The C:N and C:P ratios were significantly influenced by seawater inorganic nutrient concentrations and seasonality. Additionally, C:N ratios varied by phyla. Brown seaweeds (Ochrophyta, Phaeophyceae) had the highest mean C:N of 27.5 (range: 7.6–122.5), followed by green seaweeds (Chlorophyta) of 17.8 (6.2–54.3) and red seaweeds (Rhodophyta) of 14.8 (5.6–77.6). We used the updated C:N and C:P values to compare seaweed tissue stoichiometry with the most recently reported values for plankton community stoichiometry. Our results show that seaweeds have on average 2.8 and 4.0 times higher C:N and C:P than phytoplankton, indicating seaweeds can assimilate more carbon in their biomass for a given amount of nutrient resource. The stoichiometric comparison presented herein is central to the discourse on ocean afforestation (the deliberate replacement of phytoplankton with seaweeds to enhance the ocean biological carbon sink) by contributing to the understanding of the impact of nutrient reallocation from phytoplankton to seaweeds under large-scale seaweed cultivation.     

Conversion efficiency and nutrient digestibility of certain seaweed diets by laboratory reared Labeo rohita (Hamilton)

Impact of three different types of seaweed diets on growth, feed utilization and nutrient digestibility of L. rohita was studied for 120 days. The seaweed diet fed fishes, especially Ulva based diet showed comparatively higher growth and weight increment. Good food conversion ratio, food assimilation efficiency, protein efficiency ratio and better nutrient digestibility were recorded for seaweed diet fed fishes. The results suggests the suitability of utilizing seaweeds, Ulva fasciata, Spyridia insignis and Sargassum wightii as partial substitute for fishmeal in formulated diets of L. rohita.     

NITRITE UPTAKE TRANSIENTS AND CONSEQUENCES FOR IN VIVO ALGAL NITRATE REDUCTASE ASSAYS

We conducted in vivo nitrate reductase (NR) assays with the commercially important brown seaweed, Laminaria japonica Aresch., and present evidence that significant, sustained nitrite uptake can occur under typical assay conditions. Thus, we caution investigators to determine the importance of nitrite uptake prior to conducting similar in vivo NR assays in this and other algal species. Further, we observed high initial nitrite uptake from sea-water medium supplemented with nitrite at concentrations found in coastal seawater of Qingdao, People's Republic of China. The pattern of initial uptake was unlike that described previously as "surge uptake" or "rapid uptake" in seaweeds, but it was similar to that recently reported for rapid ammonium uptake in phytoplankton. A high initial uptake rate lasted for a few minutes, followed by a rapid decline, a transient increase and a subsequent decline to stable uptake rates after ca. 1 h. This is the first report of such initially high inorganic nitrogen uptake in seaweeds. The significance of nitrite uptake in seaweeds may have been underestimated. Seaweeds may be able to exploit short-duration nitrite pulses, as has been observed for ammonium in phytoplankton populations.     

The use of Gracilaria tikvahiae (Gracilariales,Rhodophyta) as a model system to understand the nitrogen nutrition of cultured seaweeds

Seaweeds have physiological mechanisms to acquire, utilize, and store various forms of nitrogen in environments where nitrogen levels vary tremendously in space and time. Knowledge of the nitrogen relationships of seaweeds is required for the development of successful seaweed mariculture. For example, it would seem at first that continuous nitrogen enrichment would be desirable in such systems because maximal seaweed yields are possible only when growth is not nitrogen-limited. Yet such fertilization is wasteful and can result in yield reductions due to the enhancement of epiphyte growth. Because most seaweeds can rapidly taken up high concentrations of nitrogen, far in excess of what is required for current growth demands, enrichments are needed only when internal nitrogen concentrations fall to near the critical level (i.e., the minimal tissue concentration of nitrogen required for maximal growth). Nutrients are best applied at brief pulses of high nitrogen concentrations.Dedicated to the memory of Bud Brinkhuis, friend and colleague     

210Pb dating by low background gamma counting

An intertidal fish community in Trinidad Bay, Humboldt County, California, was studied between November 1968 and May 1970. The nursery function of rocky tidal pools for juveniles of subtidal fishes was indicated by peaks in indices of seasonally abundant fish species in summer. The proportion of juveniles of seasonal species in the intertidal fish community rose significantly in collections of late spring and summer, to a maximum of 35% of the individuals collected in July. Annual dominance was relatively high due to abundant populations of the tidepool sculpin, Oligocottus maculosus, but two other fish species were also encountered in tidepools every month of the year.     

The non-native turf-forming alga Caulacanthus ustulatus displaces space-occupants but increases diversity

Non-indigenous seaweeds can be found in coastal habitats worldwide yet the ecological effects of only ~6 % of macroalgal introductions have been studied. The turf-forming red alga Caulacanthus ustulatus, a putative introduction from Asia, was discovered in southern California in 1999, yet has received very little attention despite being common in rocky intertidal habitats in the region. The purpose of this study was to evaluate the potential effects of Caulacanthus on native invertebrate and seaweed community composition. Macrofaunal, meiofaunal, and macroalgal community structure and diversity were compared between patches with (non-native) and without Caulacanthus (native) in the upper intertidal zone at 5 locations in southern California. Caulacanthus appears to displace macro invertebrates, such as barnacles, limpets, and periwinkles, while facilitating a more diverse array of meiofauna and macroalgae. This is likely due to the formation of a novel turf habitat in the upper zone where turfs are uncommon in this region naturally; algal turfs can increase habitat complexity, trap sediment, and maintain moisture during low tide which likely benefits meiofauna and seaweeds by providing food, habitat, or refuge from desiccation stress. Subsequent comparisons of invertebrate and seaweed assemblages were conducted in native and non-native patches at one site in the upper intertidal zone as well as in the middle intertidal zone where a native turf zone exists. Despite differences in community composition in the upper intertidal zone, no differences were observed in the middle zone, providing support that the novel turf created by Caulacanthus in the upper zone drives community differences.     

A meta-analysis of seaweed impacts on seagrasses: generalities and knowledge gaps

Seagrasses are important habitat-formers and ecosystem engineers that are under threat from bloom-forming seaweeds. These seaweeds have been suggested to outcompete the seagrasses, particularly when facilitated by eutrophication, causing regime shifts where green meadows and clear waters are replaced with unstable sediments, turbid waters, hypoxia, and poor habitat conditions for fishes and invertebrates. Understanding the situations under which seaweeds impact seagrasses on local patch scales can help proactive management and prevent losses at greater scales. Here, we provide a quantitative review of available published manipulative experiments (all conducted at the patch-scale), to test which attributes of seaweeds and seagrasses (e.g., their abundances, sizes, morphology, taxonomy, attachment type, or origin) influence impacts. Weighted and unweighted meta-analyses (Hedges d metric) of 59 experiments showed generally high variability in attribute-impact relationships. Our main significant findings were that (a) abundant seaweeds had stronger negative impacts on seagrasses than sparse seaweeds, (b) unattached and epiphytic seaweeds had stronger impacts than 'rooted' seaweeds, and (c) small seagrass species were more susceptible than larger species. Findings (a) and (c) were rather intuitive. It was more surprising that 'rooted' seaweeds had comparatively small impacts, particularly given that this category included the infamous invasive Caulerpa species. This result may reflect that seaweed biomass and/or shading and metabolic by-products like anoxia and sulphides could be lower for rooted seaweeds. In conclusion, our results represent simple and robust first-order generalities about seaweed impacts on seagrasses. This review also documented a limited number of primary studies. We therefore identified major knowledge gaps that need to be addressed before general predictive models on seaweed-seagrass interactions can be build, in order to effectively protect seagrass habitats from detrimental competition from seaweeds.     

Enclosure Experiments with Low-dose Additions of Phosphorus and Nitrogen in the Acidified Lake Njupfatet,Central Sweden

Experiments involving low-dose additions of phosphate, ammonium, nitrate and ADP, one by one and in combination, were performed in small (350 litre) in-situ enclosures in a moderately acid (pH 5.4) lake. Before manipulation, all large filter-feeding animals were removed by filtration. Phytoplankton responded to the nutrient additions only when both phosphorus and nitrogen were added, thus indicating a close balance between phosphorus and nitrogen limitation in the system. Variation of the inorganic nitrogen-source resulted in species-specific responses by phytoplankton. With ammonium as the nitrogen source Merismopedia tenuissima was favoured, regardless of whether this species was dominant in the phytoplankton community at the beginning of the experiment or not. With nitrate as nitrogen source Peridinium inconspicuum, which was never particularly common at the beginning of the experiments, was favoured. No other species of phytoplankton present in the bags was able to outcompete these two species as long as inorganic nutrients were added. With ADP as phosphorus source together with nitrate, a third species, Dictyosphaerium cf. botrytella, was favoured and reached dominance. The zooplankton community remaining in the bags, dominated by rotifers and calanoid nauplii, did not respond to the fertilization-induced increases in the total biomass of phytoplankton.     

RAMET DYNAMICS FOR THE CLONAL SEAWEED PTEROCLADIELLA CAPILLACEA (RHODOPHYTA): A COMPARISON WITH CHONDRUS CRISPUS AND WITH MAZZAELLA CORNUCOPIAE (GIGARTINALES)

Little is known about the dynamics and the ecological interactions among ramets (fronds) from populations of clonal red seaweeds. Small ramets are very difficult to tag, so their growth cannot be monitored directly. The temporal variation of the relationship between stand biomass and ramet density offers information on ramet performance. We calculated this relationship for an intertidal population of Pterocladiella capillacea (Gmelin) Santelices et Hommersand (Gelidiales) from Baja California, Mexico. Biomass and density were positively correlated on an annual basis, indicating that biomass accumulated without involving self-thinning among ramets. This contrasts with nonclonal seaweeds, for which self-thinning among individuals occurs during growth, but agrees with other clonal red seaweeds, such as Chondrus crispus Stackhouse and Mazzaella cornucopiae (Postels et Ruprecht) Hommersand (both Gigartinales). The growth pattern for these members of the Gelidiales and of the Gigartinales holds despite differences in holdfast morphology and ramet branching degree and despite differences in the capacity of coalescence during early stages, known only for the Gigartinales. The positive slope for the dynamic biomass–density relationship, on a bilogarithmic scale, was statistically steeper for M. cornucopiae than for P. capillacea and for C. crispus. This suggests that the addition of new ramets during the growth season may be relatively more beneficial for biomass accumulation rates for M. cornucopiae. This would be expected for high-intertidal species subjected to strong abiotic stress, for which ramet crowding constitutes a key protection. Pterocladiella capillacea occurs at the mid-intertidal zone and C. crispus at the subtidal zone, so ramets would be relatively less important in that respect.     

Effects of nitrogen and phosphorus availabilities on growth, pigment, and protein contents in Hypnea cervicornis J. Agardh (Gigartinales, Rhodophyta)

The selection of seaweed species for their use as biofilters should be based on the knowledge of their nutrient requirements and tolerance to wide variations of nutrient concentrations. Therefore, tolerance and the physiological capabilities of Hypnea cervicornis J. Agardh (Gigartinales, Rhodophyta) to growth under nitrate, ammonium, and phosphate variations and to assimilate them into soluble proteins and photosynthetic pigments were evaluated in laboratory conditions. Treatments were composed of sterilized seawater enriched with 25 % von Stosch solution (without nitrogen and phosphorus), and nitrate or ammonium and phosphate were added in combination of 100:1 and 10:1 nitrogen/phosphorus (N/P). Nitrate concentrations varied from 0 to 500 μM, and ammonium concentrations varied from 0 to 50 μM. Growth rates of H. cervicornis increased linearly with addition of ammonium, but with nitrate addition, growth varied following a saturation kinetic, and the highest growth rate (14.45 % d^?1) was observed in 200 μM of N/P ratio of 10:1. An excess of nutrients was accumulated as proteins and phycobiliproteins (mainly as allophycocyanin and phycoerythrin) at higher phosphate availability (N/P ratio of 10:1), and H. cervicornis tolerated the highest ammonium and nitrate concentrations (50 and 500 μM, respectively). These physiological responses suggest that this species could be used as biofilter for nutrient removal in eutrophicated seawater and could be cultivated in integrated multitrophic aquaculture systems.     

Differential effects of native and invasive algal wrack on macrofaunal assemblages inhabiting exposed sandy beaches

Many sandy beaches worldwide receive large amounts of drift seaweed, known as wrack, from offshore algal beds and closer rocky intertidal shores. Despite the important influence of algal wrack on macrofaunal assemblages from different coastal systems, relatively little attention has been paid to the macrofaunal responses in sandy beaches to macrophyte wrack supplies. Algal wrack is a key resource, i.e. for food and/or refuge, for beach invertebrates while its availability can affect diversity and abundance of intertidal animals including shorebirds, but the role of certain types of wrack and its location on the shore has not been examined experimentally to date. In this paper, we use experimental manipulation of two species of brown seaweeds, i.e. artificial wrack patches made up of the native macroalgae Saccorhiza polyschides and the invasive species Sargassum muticum, to test hypotheses about influences on macrofaunal assemblages inhabiting the drift line and supratidal levels of exposed beaches. Results pointed out that different types of wrack deposits were not used uniformly by invertebrates. Nutritional value differed between the two species of wrack. In most cases, the carbohydrates, lipids and organic carbon content were greater in patches of S. muticum than in patches of S. polyschides. Data also provided evidences that nutritional content and microclimatic conditions of wrack deposits, i.e. temperature and humidity, might affect macrofaunal assemblages.     

Seaweed Extracts as Biostimulants of Plant Growth and Development

Marine algal seaweed species are often regarded as an underutilized bioresource, many have been used as a source of food, industrial raw materials, and in therapeutic and botanical applications for centuries. Moreover, seaweed and seaweed-derived products have been widely used as amendments in crop production systems due to the presence of a number of plant growth-stimulating compounds. However, the biostimulatory potential of many of these products has not been fully exploited due to the lack of scientific data on growth factors present in seaweeds and their mode of action in affecting plant growth. This article provides a comprehensive review of the effect of various seaweed species and seaweed products on plant growth and development with an emphasis on the use of this renewable bioresource in sustainable agricultural systems.     

A decade of change in the seaweed hydrocolloids industry

Seaweed hydrocolloid markets continue to grow, but instead of the 3?C5% achieved in the 1980s and 1990s, the growth rate has fallen to 1?C3% per year. This growth has been largely driven by emerging markets in China, Eastern Europe, Brazil, etc. Sales of agar, alginates and carrageenans in the US and Europe are holding up reasonably well in spite of the recession. However, price increases to offset costs in 2008 and 2009 have begun to have a dampening effect on sales, especially in markets where substitution or extension with less expensive ingredients is possible. These higher prices have been driven by higher energy, chemicals and seaweed costs. The higher seaweed costs reflect seaweed shortages, particularly for carrageenan-bearing seaweeds. The Philippines and Indonesia are the dominant producers of the farmed Kappaphycus and Eucheuma species upon which the carrageenan industry depends and both countries are experiencing factors limiting seaweed production. Similar tightening of seaweed supplies are beginning to show up in brown seaweeds used for extracting alginates, and in the red seaweeds for extracting agar. The structure of the industry is also undergoing change. Producers in China are getting stronger, and while they have not yet developed the marketing skills to compete effectively in the developed world markets, they have captured much of their home market. China does not produce the red and brown seaweeds needed for higher end food hydrocolloid production. Stocking their factories with raw material has led to the supply problems. Sales growth continues to suffer from few new product development successes in recent years; although some health care applications are showing some promise, i.e., carrageenan gel capsules and alginate micro-beads.     

Mass cultivation of seaweeds: current aspects and approaches

A word-wide overview is presented of the current state of mass cultivation of seaweeds. In comparison with a total annual commercial production of fish, crustaceans and molluscs of about 120 × 10⁶t, of which one-third is produced by aquaculture, the production of seaweeds is about 10 × 10⁶t wet weight; the majoirty of this comes from culture-based systems. The Top Ten Species List is headed by the kelp Laminaria japonica with 4.2 × 10⁶t fresh weight cultivated mainly in China. The productivity of a well-developed, multi-layered, perennial seaweed vegetation is as high as dense terrestrial vegetation, and even higher annual values for productivity have been reported for tank cultures of macroalgae. Epiphytes provide a major problem for the seaweed cultivator, but can be controlled by growing plants at high densities in rope cultures in the sea, or, more easily, in seaweed tank cultures on land. The main environmental problem of animal (fed) aquaculture is the discharge of nutrient loads into coastal waters, e.g., 35 kg N and 7 kg P t^–1 aquacultured fish. Integration of fish and seaweed farming may help to solve this problem, since seaweeds can remove up to 90% of the nutrient discharge from an intensive fish farm. Mass culture of commercially valuable seaweed species is likely to play an increasingly important role as a nutrient-removal system to alleviate eutrophication problems due to fed aquaculture.     

Water-borne cues of a non-indigenous seaweed mediate grazer-deterrent responses in native seaweeds, but not vice versa

Plants optimise their resistance to herbivores by regulating deterrent responses on demand. Induction of anti-herbivory defences can occur directly in grazed plants or from emission of risk cues to the environment, which modifies interactions of adjacent plants with, for instance, their consumers. This study confirmed the induction of anti-herbivory responses by water-borne risk cues between adjoining con-specific seaweeds and firstly examined whether plant-plant signalling also exists among adjacent hetero-specific seaweeds. Furthermore, differential abilities and geographic variation in plant-plant signalling by a non-indigenous seaweed as well as native seaweeds were assessed. Twelve-day induction experiments using the non-indigenous seaweed Sargassum muticum were conducted in the laboratory in Portugal and Germany with one local con-familiar (Portugal: Cystoseira humilis, Germany: Halidrys siliquosa) and hetero-familiar native species (Portugal: Fucus spiralis, Germany: F. vesiculosus). All seaweeds were grazed by a local isopod species (Portugal: Stenosoma nadejda, Germany: Idotea baltica) and were positioned upstream of con- and hetero-specific seaweeds. Grazing-induced modification in seaweed traits were tested in three-day feeding assays between cue-exposed and cue-free (?=?control) pieces of both fresh and reconstituted seaweeds. Both Fucus species reduced their palatability when positioned downstream of isopod-grazed con-specifics. Yet, the palatability of non-indigenous S. muticum remained constant in the presence of upstream grazed con-specifics and native hetero-specifics. In contrast, both con-familiar (but neither hetero-familiar) native species reduced palatability when located downstream of grazed S. muticum. Similar patterns of grazer-deterrent responses to water-borne cues were observed on both European shores, and were almost identical between assays using fresh and reconstituted seaweeds. Hence, seaweeds may use plant-plant signalling to optimise chemical resistance to consumers, though this ability appeared to be species-specific. Furthermore, this study suggests that native species may benefit more than a non-indigenous species from water-borne cue mediated reduction in consumption as only natives responded to signals emitted by hetero-specifics.