Nutrient Addition to Experimental Rocky Shore Communities Revisited: Delayed Responses, Rapid Recovery

Coastal eutrophication may alter the dominance patterns of marine macroalgae, with potential consequences for the associated fauna and the entire ecosystem. Benthic macroalgae and animals in control and nutrient-enriched mesocosms were monitored to investigate eutrophication-induced changes in rocky shore communities. During a 3-year project, nutrient addition had only minor effects on the community structure, such as increased cover and biomass of green Ulva spp. and increased abundance of certain animal species at high nutrient levels. This study is a 4-year extension of a previously reported project, with 2 extra years of effect studies (altogether 5 years) and a subsequent 2 years for recovery. During the 4th year of nutrient enrichment, the cover of Fucus vesiculosus and Fucus serratus started to decline. In the 5th year, these canopy species crashed and there was an evident take-over by green algae at high nutrient addition levels. The previously observed abundance stimulation for fauna disappeared later in the time series, probably due to the loss of the macroalgal canopy. After less than 2 years on regular seawater, the algal and animal communities had returned to within the range of normal variability. The results indicate that established rocky shore communities of perennial algae with associated fauna are able to persist for several years, even at very high nutrient levels, but that community shifts may suddenly occur if eutrophication continues. They also indicate that rocky shore communities have the ability to return rapidly to natural undisturbed conditions after the termination of nutrient enhancement.     

The Response of Experimental Rocky Shore Communities to Nutrient Additions

The aim of this study was to determine whether the experimental nutrient enrichment of littoral rocky shore communities would be followed by a predicted accumulation of fast-growing opportunistic algae and a subsequent loss of perennial benthic vegetation. Inorganic nitrogen (N) and potassium (P) was added to eight concrete mesocosms inhabited by established littoral communities dominated by fucoids. The response to nutrient enrichment was followed for almost 2 1/2 years. Fast-growing opportunistic algae (periphyton and ephemeral green algae) grew significantly faster in response to nutrient enrichment, but the growth of red filamentous algae and large perennial brown algae was unaffected. However, these changes were not followed by comparable changes in the biomass and composition of the macroalgae. The biomass of opportunistic algae was stimulated only marginally by the nutrient enrichment, and perennial brown algae (fucoids) remained dominant in the mesocosm regardless of nutrient treatment level. Established rocky shore communities thus seem able to resist the effects of heavy nutrient loading. We found that the combined effects of the heavy competition for space and light imposed by canopy-forming algae, preferential grazing on opportunistic algae by herbivores, and physical disturbance, succeeded by a marked export of detached opportunistic algae, prevented the fast-growing algae from becoming dominant. However, recruitment studies showed that the opportunistic algae would become dominant when free space was available under conditions of high nutrient loading and low grazing pressure. These results show that established communities of perennial algae and associated fauna in rocky shore environments can prevent or delay the accumulation of bloom-forming opportunistic algae and that the replacement of long-lived macroalgae by opportunistic species at high nutrient loading may be a slow process. Nutrient enrichment may not, in itself, be enough to stimulate structural changes in rocky shore communities.     

Response on scallop culture to enhanced nutrient supply by experimental fertilisation of a landlocked bay

The effect of an enhanced nutrient supply to coastal waters of a landlocked bay, Hopavågen in Central Norway, on the phytoplankton production and biomass, and on growth of scallops (Pecten maximus) was studied in 1997–1999. Nitrogen, silicon and phosphorous (N:Si:P = 16:8:1, atomic) were added daily between May and October in 1998 at a level of 0.4 mg P m^–3 day^–1. The concentration of nutrient addition was doubled in 1999 during the same period. High addition of nutrients (1999) resulted in a significantly higher phytoplankton biomass in the summer period, expressed as chlorophyll a content, than without nutrient (1997) and low nutrient (1998). The respective mean chlorophyll a levels were 2.4 in 1999, 1.6 in 1998 and 1.2 g l^–1 in 1997. The mean primary production during the summers generally increased with the addition of nutrients from an average level of 320 mg carbon m^–2 day^–1 in 1997 to 1200 mg carbon m^–2 day^–1 in 1999. Scallops placed at 10 m depth in Hopavågen showed an increase in growth rate of the outer scallop shell in the period July–September from 0.16% day^–1 in 1997 to 0.53% day^–1 in 1998. Scallops grown in an unfertilised control station in the fjord outside Hopavågen had a significantly lower growth rate than those grown in the fertilised water of Hopavågen. The results showed decreased growth rate with increasing shell sizes. However, for all size groups studied a higher growth rate of the scallops was observed when nutrients were added to the bay. The tissue dry weight content of scallops grown in Hopavågen was 2–4 times higher than in the control scallops.     

Effect of nutrient supply on growth of blue mussels (Mytilus edulis) in a landlocked bay

The growth of blue mussels (Mytilus edulis) was studied in the landlocked bay Hopavågen in central Norway for 3 years, of which in 2 years (1998 and 1999) nutrients were added to increase the primary production. Nutrients (N:Si:P) were added daily from May to October in 1998 (molar ratio 15:5:4.1) and 1999 (molar ratio 16:8:1). The doses of nutrients correspond to 0.4 and 0.8 g P l^–1 day^–1 in 1998 and 1999 respectively. The growth of blue mussels (Mytilus edulis) in 1997 was followed at four depths (1, 2, 4 and 7 m). In 1998 and 1999 growth was followed at 2 and 10 m depths at four locations in Hopavågen and at a control station outside on the coast. The nutrient supply in 1998 only slightly increased the algal biomass (chlorophyll a), whereas mean daily primary production during the summer remained at the same level as the previous year. The increased nutrient supply in 1999 caused a nearly 50 and 100% increase in mean summer biomass and daily primary production, respectively. The growth of blue mussels in Hopavågen in 1997 and 1998 was within the same size range during the summer. In 1999 the shell length of blue mussels kept at 2 m depth was significantly higher than in the previous year at end of the growth season. The recorded growth was also significant higher than for mussels at 2 m depth at the control station. No difference in shell length was observed on mussels grown at 10 m depth in Hopavågen and in the control stations in 1998 and 1999. A higher tissue content was found in blue mussels grown at 2 m depth in Hopavågen, both in 1998 and 1999 when compared to the control groups. At 10 m depth no differences were recorded.     

Responses to nutrient enrichment,wave action and disturbance in rocky shore communities

A high degree of resistance against nutrient enrichment has previously been demonstrated for macroalgal-dominated rocky shore communities in the presence of moderate to large amounts of macroinvertebrate grazers. To experimentally examine, under controlled conditions, the possible roles for this resistance of two other factors, i.e. disturbance (presence/absence of the macroalgal canopy itself) and wave action, the canopy algae and associated algal and animal assemblages were removed by scraping from approximately one third of the area of eight littoral mesocosms, subjected to two different wave action regimes. After this, excessive nutrients were added to four mesocosms with the factor nutrients fully crossed with the factor wave action with two replicate mesocosm basins of each nutrient/wave treatment combination. Disturbance was added to the design as a within-basin factor thus making up a split-plot experiment. The abundance of grazers was allowed to vary freely and under the influence of the treatments. After 11 summer weeks, there were significant differences in community structure between nutrient enrichment levels for both algal and animal assemblages when examined by multivariate statistical techniques. Univariate analyses confirmed a significantly stimulated colonisation by green algae, mainly Ulva lactuca, in both disturbed (scraped) and undisturbed areas of nutrient-enriched mesocosms. In un-enriched mesocosms, the green algae were absent from undisturbed areas and rare in disturbed areas, where mainly brown Ectocarpus spp. and red algae had settled. Among the macrofauna, the total abundance of grazers was stimulated in nutrient-enriched mesocosms with individuals of the amphipod genus Gammarus and the isopod genus Jaera being especially numerous. With regard to wave action, no significant differences occurred in community structure, although there were indications of significant nutrient × wave effects for both the amount of exported red algae and the amount of accumulated brown algae. The study shows that eutrophication-related community shifts on rocky shores may occur very rapidly, regardless of the level of wave-energetic stress and the abundance of grazers, if the nutrient concentrations are high and the colonisation and growth of opportunistic algae are facilitated by disturbance such as (naturally or anthropogenically driven) canopy gap forming processes.     

Responses to nutrient enrichment,wave action and disturbance in rocky shore communities

A high degree of resistance against nutrient enrichment has previously been demonstrated for macroalgal-dominated rocky shore communities in the presence of moderate to large amounts of macroinvertebrate grazers. To experimentally examine, under controlled conditions, the possible roles for this resistance of two other factors, i.e. disturbance (presence/absence of the macroalgal canopy itself) and wave action, the canopy algae and associated algal and animal assemblages were removed by scraping from approximately one third of the area of eight littoral mesocosms, subjected to two different wave action regimes. After this, excessive nutrients were added to four mesocosms with the factor nutrients fully crossed with the factor wave action with two replicate mesocosm basins of each nutrient/wave treatment combination. Disturbance was added to the design as a within-basin factor thus making up a split-plot experiment. The abundance of grazers was allowed to vary freely and under the influence of the treatments. After 11 summer weeks, there were significant differences in community structure between nutrient enrichment levels for both algal and animal assemblages when examined by multivariate statistical techniques. Univariate analyses confirmed a significantly stimulated colonisation by green algae, mainly Ulva lactuca, in both disturbed (scraped) and undisturbed areas of nutrient-enriched mesocosms. In un-enriched mesocosms, the green algae were absent from undisturbed areas and rare in disturbed areas, where mainly brown Ectocarpus spp. and red algae had settled. Among the macrofauna, the total abundance of grazers was stimulated in nutrient-enriched mesocosms with individuals of the amphipod genus Gammarus and the isopod genus Jaera being especially numerous. With regard to wave action, no significant differences occurred in community structure, although there were indications of significant nutrient × wave effects for both the amount of exported red algae and the amount of accumulated brown algae. The study shows that eutrophication-related community shifts on rocky shores may occur very rapidly, regardless of the level of wave-energetic stress and the abundance of grazers, if the nutrient concentrations are high and the colonisation and growth of opportunistic algae are facilitated by disturbance such as (naturally or anthropogenically driven) canopy gap forming processes.     

Responses to fish predation and nutrients by plankton at different levels of taxonomic resolution

1. To improve mechanistic understanding of plankton responses to eutrophication, a mesocosm experiment was performed in the shallow littoral zone of a south Swedish lake, in which nutrient and fish gradients were crossed in a fully factorial design. 2. Food chain theory accurately predicted total biomass development of both phyto‐ and zooplankton. However, separating zooplankton and algae into finer taxonomic groups revealed a variety of responses to both nutrient and fish gradients. 3. That both nutrients and fish are important for phytoplankton dynamics was seen more clearly when viewing each algal group separately, than drawing conclusions only from broad system variables such as chlorophyll a concentration or total phytoplankton biovolume. 4. In some taxa, physiological constraints (e.g. sensitivity to high pH and low concentrations of free CO₂) and differences in competitive ability may be more important for the biomass development than fish predation, grazing by herbivorous zooplankton, and nutrient availability. 5. We conclude that food chain theory accurately predicted responses in system variables, such as total zooplankton or algal biomass, which are shaped by the dynamics of certain strong interactors (‘keystone species’), such as large cladocerans, cyanobacteria and edible algae (<50 μm), whereas responses at finer taxonomic levels cannot be predicted from current theory.     

High Organic Carbon Export Precludes Eutrophication Responses in Experimental Rocky Shore Communities

We studied the effect of nutrient inputs on the carbon (C) budget of rocky shore communities using a set of eight large experimental mesocosms. The mesocosms received a range of inorganic nitrogen (N) and phosphorus (P) additions, at an N:P ratio of 16. These additions were designed to elevate the background concentration, relative to that in eutrophic Oslofjord (Norway) waters, by 1, 2, 4, 8, 16, 32 μmol dissolved inorganic nitrogen (DIN)l⁻¹ (and the corresponding P increase). Two unamended mesocosms were used as controls. The nutrients were added continuously for 27 months before gross primary production (GPP), respiration (R), net community production (NCP), and dissolved organic carbon (DOC) production were assessed for the dominant algal species (Fucus serratus) and for the whole experimental ecosystem. Inputs and outputs of DOC and particulate organic carbon (POC) from the mesocosms were also quantified. The F. serratus communities were generally autotrophic (average P/R ratio = 1.33 ± 0.12), with the GPP independent of the nutrient inputs to the mesocosms, and maintained a high net DOC production during both day (0.026 ± 0.008 g C m⁻² h⁻¹) and night (0.015 ± 0.004 g C m⁻² h⁻¹). All the experimental rocky shore ecosystems were autotrophic (P/R ratio = 2.04 ± 0.28), and neither macroalgal biomass nor production varied significantly with increasing nutrient inputs. Most of the excess production from these autotrophic ecosystems was exported from the systems as DOC, which accounted for 69% and 58% of the NCP of the dominant community and the experimental ecosystem, respectively, the rest being lost as POC. High DOC release and subsequent export from the highly energetic environments occupied by rocky shore communities may prevent the development of eutrophication symptoms and render these communities resistant to eutrophication. Received 10 October 2001; accepted 18 July 2002.     

Are rocky shore ecosystems affected by nutrient-enriched seawater? Some preliminary results from a mesocosm experiment

The response of rocky shore ecosystems to increased nutrient availability was examined in eight land-based mesocosms designed for hard-bottom littoral communities built at Marine Research Station Solbergstrand (Norway). The average seawater volume in each basin was 9 m³ with an average water residence time of about 2 h. A tidal regime resembling that in the fjord was maintained in the basins, and waves were generated regularly. NH₄NO₃ and H₃PO₄, at a constant molar NP ratio of 16:1, was added into 6 basins at concentrations 1, 2, 4, 8, 16, 32 M DIN above the background DIN concentration during 1 1/2 years. Two mesocosms were kept as control treatment. Marine communities were introduced into the basins two years prior to the start of nutrient dosage. The effects of nutrient enrichment were few and only marginal during the first year of nutrient addition, while some effects became more obvious during the second year. The growth rate of the periphyton and fast-growing macroalgae communities was stimulated by nutrient enrichment, while the response was less evident among the perennial fucoids. The structure of the macroalgal communities, however, did not change during 16 months' measurements. In contrast, growth on artificial rock substrates during the same period of time revealed intensive growth of the fast-growing Ulva lactuca in high-dosed basins compared with low-dosed and control basins, which were dominated by the fucoid Fucus serratus. The fauna communities exhibited only a minor response to nutrient treatment. The common periwinkle Littorina littorea, however, appeared with increased abundance in the high-dosed basins. The total system metabolism tended to increase slightly, but not significantly, with increased nutrient loading.     

Moving on up: can results from simple aquatic mesocosm experiments be applied across broad spatial scales?

1. Aquatic ecologists use mesocosm experiments to understand mechanisms driving ecological processes. Comparisons across experiments, and extrapolations to larger scales, are complicated by the use of mesocosms with varying dimensions. We conducted a mesocosm experiment over a volumetric scale spanning five orders of magnitude (from 4 L to whole ponds) to determine the generality of algal responses to nutrient enrichment. Recognising that mesocosm dimensions may affect algal growth, we also manipulated the ratio of mesocosm surface area to volume (SA : V) over two levels (high versus low). We used mesocosm tanks of similar size and construction to those commonly used in aquatic experiments to increase the generality of our results. 2. Volume was generally a stronger determinant of algal responses than mesocosm shape (i.e. SA : V). However, the effects of both volume and shape on algae were weak and explained a small portion of the variance in response variables. In addition, there was no consistent, directional relationship (positive or neutral) between mesocosm volume and algal abundance (estimated by chlorophyll concentration). Combined, our findings suggest that results from small‐scale experiments, examining the direct response of algae to nutrient enrichment, can probably be ‘moved on up’ and applied to larger, more natural aquatic systems. 3. Algal response to nutrient enrichment (e.g. nutrient use efficiency and effect size) varied strongly with time. This underscores the importance of choosing an experimental timescale appropriate to the biological and/or ecological process of interest. 4. We compared our results to those from a recent meta‐analysis of nutrient‐limitation studies that included 359 freshwater pelagic experiments, spanning a wide range of volumetric and temporal scales. Similar findings between this experiment and the meta‐analysis indicate that algal response to nutrient enrichment varies little across spatial scales. Therefore, it is probable that results from small‐scale pelagic algal nutrient‐limitation experiments are relevant to large‐scale processes, such as eutrophication.     

Variable responses within epiphytic and benthic microalgal communities to nutrient enrichment

We evaluated how changes in nutrient supply altered the composition of epiphytic and benthic microalgal communities in a Thalassia testudinum (turtle grass) bed in Florida Bay. We established study plots at four sites in the bay and added nitrogen (N) and phosphorus (P) to the sediments in a factorial design. After 18, 24, and 30 months of fertilization we measured the pigment concentrations in the epiphytic and benthic microalgal assemblages using high performance liquid chromatography. Overall, the epiphytic assemblage was P-limited in the eastern portion of the bay, but each phototrophic group displayed unique spatial and temporal responses to N and P addition. Epiphytic chlorophyll a, an indicator of total microalgal load, and epiphytic fucoxanthin, an indicator of diatoms, increased in response to P addition at one eastern bay site, decreased at another eastern bay site, and were not affected by P or N addition at two western bay sites. Epiphytic zeaxanthin, an indicator of the cyanobacterial/coralline red algae complex, and epiphytic chlorophyll b, an indicator of green algae, generally increased in response to P addition at both eastern bay sites but did not respond to P or N addition in the western bay. Benthic chlorophyll a, chlorophyll b, fucoxanthin, and zeaxanthin showed complex responses to N and P addition in the eastern bay, suggesting that the benthic assemblage is limited by both N and P. Benthic assemblages in the western bay were variable over time and displayed few responses to N or P addition. The contrasting nutrient limitation patterns between the epiphytic and benthic communities in the eastern bay suggest that altering nutrient input to the bay, as might occur during Everglades restoration, can shift microalgal community structure, which may subsequently alter food web support for upper trophic levels.     

Temporal comparison of the composition and zonation of rocky intertidal organisms at Cocos Island National Park, Pacific, Costa Rica

Several biological and physical factors change the rocky shore communities. The desiccation time and the tolerance of the intertidal species produce the vertical zonation. In many studies around the world, a temporal change in this zonation is presented.In Costa Rica, only studies that include temporal trends were carried out in Punta Mala and Montezuma, Pacific coast in 80's. The rocky intertidal of the Cocos Island National Park, Costa Rica were surveyed photographically. The Chatham bay was sampled in three expeditions (January 2007, October 2007 and April 2008). Photos corresponding to 25x25cm quadrats were taken with the goal to determine diversity and composition differences in rocky shore organisms between sampling dates. The Wafer bay was sampled in January and October 2007. The intertidal of Chatham consists of basaltic rock, while Wafer has basaltic and ignimbrite boulders. The main difference between sites were the higher algae cover (erect-frondose forms) and number of organism bands at Chatham bay. Temporal change was not found in the total cover of sessile fauna and autotrophs. The barnacle Tetraclita stalactifera, that occurs above the algal fringe (lower intertidal), was the invertebrate with the highest coverage. The mobile fauna biodiversity presented no significant trend between sampled months. However, the identity of species, their cover and their abundance showed a moderate temporal change. In October 2007, when the sea surface temperature was 23 degrees C the infralittoral zone had an increase in green algae cover. The red algae (crust and erect-frondose forms) were dominant in January and April. The pulmonate limpet, Siphonaria gigas and a bacterial biofilm at mid littoral showed a negative association. The snails of the high littoral and the supralittoral zone showed a temporal change in their abundance, but with contrasting patterns between sites. The temporal variation in the assemblages increased from the supralittoral to the infralittoral possibly due to changes in the water temperature and climatic conditions, that could influence the intertidal zone during the high and low tide, respectively.     

Size-specific effects of increased sediment loads on gastropod communities in Lake Tanganyika, Africa

The remarkable biodiversity of the littoral zone of Lake Tanganyika appears to be at risk through increasing sediment input caused by anthropogenic pressures. An in-situ field experiment was done to investigate the effects of increased sediment loads on the size-structure of gastropod communities on a rocky shore site in the lake. Gastropods were removed prior to the addition of sediment, and subsequent further removals were done after seven days and six months. Relative to controls, mean size of individuals of both Lavigeria grandis (Smith) and Lavigeria sp. Q decreased following the addition of sediment, while that of Lavigeria sp. P increased. A subsequent laboratory experiment found that survivorship of larger individuals of Lavigeria grandis was less than smaller individuals in the sediment-impacted treatment. It is hypothesised that decreased algal availability on sediment-impacted rock surface increased starvation of larger snails. Alterations to the size-structure of Lavigeria sp. P populations appear related to intraspecific differences in competitive ability. These results highlight the complex and unpredictable effects of increases in sediment loads on aquatic ecosystems and have implications for the conservation of littoral communities in sediment-impacted areas.     

Long-term changes in the benthic communities of the Pomeranian Bay (Southern Baltic Sea)

Long-term changes in the macrofauna of the Pomeranian Bay were studied by comparing survey data from the 1950s, 1980s, and 1990s. The study area has undergone significant eutrophication during the period of investigation. Biomass of filter-feeding bivalves increased significantly. Spatial distribution patterns of several species have changed. Strong decreases in species richness were caused by oxygen depletion at stations deeper than 15 m.Saduria entomon, Monoporeia affinis, andPontoporeia femorata vanished entirely between 1981 and 1993. Although a causal relationship between simultaneous increases of nutrient levels and macrobenthic biomass cannot be verified, eutrophication is proposed to be the major process affecing changes in macrofauna assemblages. In addition, changes in hydrography and climate increased frequency and severity of oxygen depletion events in the Pomeranian Bay since the mid 1980s.     

Responses of growth and phlorotannins in <Emphasis Type="Italic">Fucus vesiculosus</Emphasis> to nutrient enrichment and herbivory

Discharge from anthropogenic sources may modify both macroalgal growth patterns and resource allocation to carbon based secondary compounds, thereby affecting their susceptibility for herbivory. We tested the effect of eutrophication in terms of nutrient enhancement on growth and phlorotannin concentration of Fucus vesiculosus by conducting manipulative experiments in the field and mesocosms. In the field experiment we utilised fish farms as nutrient sources and in the mesocosm-experiment we manipulated ambient nutrient levels and occurrence of the herbivorous isopod Idotea baltica. Vicinity of a fish farm affected neither growth nor the phlorotannin concentration of Fucus but increased the amount of epiphytes growing on Fucus. Other organisms such as epiphytic filamentous algae and periphyton, which are more capable of quickly utilizing excess nutrients, may restrain the direct effects of nutrient enhancement on Fucus. In a manipulative mesocosm experiment, neither nutrient enrichment nor occurrence of herbivores affected phlorotannin concentration implying lack of induced defences, at least in terms of increasing phlorotannin concentration. Feeding of thallus decreased the growth rate of algae, but the number of reproductive organs, receptacles, was not affected by herbivory. The negative effect of herbivory on the amount of apical tips tended to be stronger under nutrient enriched conditions. We conclude that eutrophication processes, in terms of nutrient enrichment, does not have strong direct effect on growth or phlorotannin production of F. vesiculosus. However, there may be important indirect consequences. First, herbivory may be targeted more to apical parts of the thallus under eutrophicated conditions. Second, the result that Fucus growing close to nutrient sources were smaller than those in control areas may reflect differences in mortality schedules of algae between eutrophicated and control areas.     

Ecological sensitivity of marl lakes to nutrient enrichment: evidence from Hawes Water,UK

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Eutrophication-induced changes in benthic algae affect the behaviour and fitness of the marine amphipod Gammarus locusta

This study, conducted in mesocosms, natural field sites, and in laboratory aquaria, showed that eutrophication altered the nutrient status and dominance patterns among marine macroalgae, which in turn, stimulated gammaridean density. Gammaridean abundance correlated positively with both nutrient addition and the amount of green algae (also stimulated by nutrient enrichment). Path analysis indicated that the direct effect of nutrients on gammaridean density was of less importance than the indirect effect through increased production of green algae. In cage colonisation experiments, either in the field or in a control mesocosm kept under ambient nutrient conditions, more gammarids colonised nutrient enriched algae (E-algae) than algae with ambient nutrient levels (A-algae). Gammarus locusta generally grew faster on nutrient enriched algal specimens and when reared on green rather than on brown algae (fucoids). The nutrient status of periphytic algae did not affect gammaridean growth significantly, but the number of egg-carrying females (and thus egg production) was significantly higher among gammarids reared on E-periphyton. The gammaridean habitat preference order (red > green > brown > periphyton) was almost the reverse of their growth rate in feeding assays (periphyton > green > brown). This implies that macroalgae may be more important as a habitat than as a food source for these animals, which then have to become mobile in search of optimal food items. In this process, algal nutrient content was important as the gammarids in our study actively chose high quality nutrient-rich food, which, in addition, increased their fitness. Stimulated growth rates and egg production may ultimately lead to population increase, which, combined with the preference for high nutrient food items may dampen the initial effect of nutrient enrichment (i.e. blooms of green macroalgae) in shallow coastal waters.     

Benthic community responses to nutrient enrichment and predator exclusion: Influence of background nutrient concentrations and interactive effects

Potential community effects of nutrient enhancement are a topic of theoretical interest and increasing management concern in coastal marine systems. While increased nutrient levels may lead to increased microalgal production and biomass, studies have provided variable evidence regarding the existence of upward cascade effects on macrofauna. In benthic marine communities, limitation by predation or factors preventing recruitment response may contribute to weak coupling between resource availability and macrobenthos abundances. We conducted blocked nutrient addition and predator exclusion experiments in the intertidal of two estuaries that varied in background nutrient concentrations (Cape Fear and White Oak, southeastern North Carolina). Benthic community comparisons were also made among these and two other North Carolina estuaries to examine correlations in distribution patterns. Cape Fear, which had the highest background nitrogen and phosphorus concentrations, also had highest ambient benthic microalgal biomass. There was no significant response of microalgal biomass to local nutrient additions in Cape Fear and only one macrofaunal taxon during one season exhibited abundance responses to nutrient additions. White Oak, with lower background nutrient levels, was characterized by significant microalgal responses to nutrient additions and significant macrofauna abundance responses for 50% of the species examined during summer experiments. However, all of these macrofauna declined in abundance with nutrient enhancement while biomass remained constant or significantly increased with nutrient additions. This suggests a complex response of macrofauna to nutrient additions in this estuary with greater biomass per individual but a corresponding decline in abundances. Top-down/bottom-up interactive effects were observed for haustoriid amphipods, which were uncommon or absent when predators had access, but exhibited strong biomass responses to nutrient enhancement when predators were excluded. These results support a growing body of literature that indicates the importance of background conditions in regulating benthic community responses to nutrient enhancement. However, responses may be complex with biomass per individual rather than densities being the primary response variable for some taxa and predator moderation of responses occurring for some taxa but not others.     

Sampling strategies for biotope definition: minimal sampling area for selected groups of macroinvertebrates in the rocky subtidal of São Miguel, Azores

Azorean rocky shores are mainly characterized by patchy algae-based communities with variable associated macrofauna. Characterization studies should therefore include quantitative information for both algae and macroinvertebrates. Unlike for the algae, minimal sampling areas are undefined for macroinvertebrates in the Azores. The present study defines the minimal area to be used for the assessment of the abundance of conspicuous benthic macroinvertebrate abundance. This study proposes methodologies to be used for a selected group of invertebrates when simultaneously undertaking quantifications of macroalgae.     

Homogenization of fish assemblages in different lake depth strata at local and regional scales

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