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1.
We conducted a field experiment to assess the response of phytal harpacticoids to nutrient‐driven increases of epiphyte load in Posidonia oceanica meadows. First, we evaluated differences in species richness, diversity and assemblage structure of phytal harpacticoids in P. oceanica meadows with differing epiphyte loads. Secondly, we conducted a field experiment where epiphyte load was increased through an in situ addition of nutrients to the water column and evaluated the responses of the harpacticoid assemblages. We predicted that there would be changes in the harpacticoid assemblages as a result of nutrient‐driven increases of epiphyte load, and that these changes would be of a larger magnitude in meadows of low epiphyte load. Our results show that the harpacticoid fauna (>500 μm) present in P. oceanica meadows in the Bay of Palma comprised taxa which are considered phytal and other less abundant ones previously described as sediment dwellers or commensal on other invertebrate species. Nutrient addition had an overall significant effect on epiphyte biomass and on harpacticoid abundance, diversity and assemblage structure, possibly as a response to the increased resources and habitat complexity provided by epiphytes. The abundance of dominant species at each location was favoured by nutrient addition and in some cases correlated with epiphytic biomass, although never strongly. This may indicate that structural complexity or diversity of the epiphytic cover might be more important than the actual epiphytic biomass for the harpacticoid species investigated. More species‐specific studies are necessary to ascertain this and clarify the relationships between harpacticoids and epiphytes in seagrass meadows. To our knowledge, this is the first account of harpacticoid species associated with P. oceanica leaves and the epiphytic community they harbour in the Mediterranean Sea.  相似文献   

2.
This paper reviews the main studies dealing with epiphytes of both leaves and rhizomes of Posidonia oceanica, the main seagrass found in the Mediterranean Sea. A total of 660 epiphyte species has been recorded, including 430 on leaves and 437 on rhizomes. Of these, 205 are Rhodophyta, 59 Ochrophyta, 43 Chlorophyta, 78 Porifera, 82 Cnidaria, 19 Annelida, 155 Bryozoa and 19 Tunicata. The epiphyte assemblages demonstrate a gradient of dissimilarity from west to east, with the eastern area being the most unlike the others. The differences can be attributed mostly to rare species that differ amongst the Mediterranean regions. Many of the dominant taxa have been found in all meadows studied. Data concerning species composition, abundance, and patterns of spatial and temporal variability are summarized, and biotic and abiotic factors controlling the structure of assemblages are discussed. Responses of epiphyte assemblages to anthropogenic stressors, such as nutrient enrichment and biological invasions, are discussed in relation to the use of epiphytes as ecological indicators.  相似文献   

3.
Seagrass beds occur in various morphological forms, ranging from small patches to continuous meadows. The endemic Mediterranean seagrass Posidonia oceanica forms dense and extensive stands that occur in several different morphotypes, including reticulate (seagrass interspersed with a different habitat type, such as bare sand) and continuous beds. This study, undertaken in the Maltese Islands, examined whether reticulate and continuous P. oceanica beds, located adjacent to each other and at similar depths, had different within-bed architectural characteristics. Five commonly used architectural measures (shoot density, number of leaves per shoot, mean leaf length, mean leaf width and shoot biomass) were measured from P. oceanica shoots collected from the two bed types at three different spatial scales: (1) tens of metres (‘small’ scale); (2) hundreds of metres (‘medium’ scale); and (3) kilometres (‘large’ scale). Results of 2-factor ANOVA (factor 1=bed type; factor 2=sampling locality) carried out at the three spatial scales indicated significant differences between the two bed types in shoot density (P<0.01) and leaf length (P<0.05) at the small scale, and in leaf number (P<0.05) at the large scale. Significant interactions were also apparent for shoot density (at the large scale) and for shoot biomass (at the medium scale). However, the results obtained did not indicate consistent architectural differences between the two P. oceanica bed types over the spatial scales considered. Spatial variations in within-bed architectural characteristics observed were therefore thought to be attributable mainly to the influence of local environmental factors. The findings are discussed with reference to the conservation and management of P. oceanica habitat.  相似文献   

4.
The Malta‐Comino Channel (Maltese islands, central Mediterranean), supports extensive meadows of the seagrass Posidonia oceanica that in some places extend to a depth of around 43 m, which is rare for this seagrass. To assess spatial and temporal variation in the state of the deeper parts of the P. oceanica meadow with time, data on the structural characteristics of the seagrass meadow at its lower bathymetric limit were collected during the summers of 2001, 2003 and 2004 from four stations (two stations within each of two sites) located at a similar depth, over a spatial extent of 500 m. Shoot density was estimated in situ, while data on plant architecture (number of leaves, mean leaf length, and epiphyte load) were successfully obtained using an underwater photographic technique that was specifically designed to avoid destructive sampling of the seagrass. Results indicated that P. oceanica shoot density was lower than that recorded from the same meadow during a study undertaken in 1995; the observed decrease was attributed to the activities of an offshore aquaculture farm that operated during the period 1995–2000 in the vicinity of the meadow. ANOVA indicated significant spatial and temporal variations in meadow structural attributes at both sites during the 3‐year study; for example, shoot density values increased overall with time at site A; a indication of potential recovery of the meadow following cessation of the aquaculture operations. Lower shoot density values recorded from site B (compared with site A) were attributed to higher epiphyte loads on the seagrass, relative to those at site A. The findings, which include new data on the structural characteristics of P. oceanica occurring at depths >40 m, are discussed with reference to the use of the non‐destructive photographic technique to monitor the state of health of deep water seagrass meadows.  相似文献   

5.
Leaf mechanical traits are important to understand how aquatic plants fracture and deform when subjected to abiotic (currents or waves) or biotic (herbivory attack) mechanical forces. The likely occurrence of variation during leaf ontogeny in these traits may thus have implications for hydrodynamic performance and vulnerability to herbivory damage, and may be associated with changes in morphologic and chemical traits. Seagrasses, marine flowering plants, consist of shoot bundles holding several leaves with different developmental stages, in which outer older leaves protect inner younger leaves. In this study we examined the long‐lived seagrass Posidonia oceanica to determine ontogenic variation in mechanical traits across leaf position within a shoot, representing different developmental stages. Moreover, we investigated whether or not the collection procedure (classical uprooted shoot versus non‐destructive shoot method: cutting the shoot without a portion of rhizome) and time span after collection influence mechanical measurements. Neither collection procedure nor time elapsed within 48 h of collection affected measurements of leaf biomechanical traits when seagrass shoots were kept moist in dark cool conditions. Ontogenic variation in mechanical traits in P. oceanica leaves over intermediate and adult developmental stages was observed: leaves weakened and lost stiffness with aging, while mid‐aged leaves (the longest and thickest ones) were able to withstand higher breaking forces. In addition, younger leaves had higher nitrogen content and lower fiber content than older leaves. The observed patterns may explain fine‐scale within‐shoot ecological processes of leaves at different developmental stages, such as leaf shedding and herbivory consumption in P. oceanica.  相似文献   

6.
This paper investigates patterns of variability in epiphytes of Posidonia oceanica leaves at various spatial scales around Sicily, including geographical differences among the Mediterranean basins, differences between the small islands and mainland, and the variability among and within replicated meadows in each of the previous conditions. Data on percentage cover of the most common epiphytic organisms were analysed by univariate techniques. Encrusting red algae, encrusting brown algae, filamentous algae, encrusting bryozoans, erect bryozoans, hydroids and Foraminifera were the most abundant taxa. Significant differences in the abundance of taxa were detected among geographical regions, while no significant differences were found between the islands and mainland. At smaller scales, variability was concentrated mostly among leaves 100s of centimetres apart and among meadows a few kilometres apart. These results suggest that both geographical and local processes are important in structuring the epiphytes of P. oceanica leaves in this area of the Mediterranean.  相似文献   

7.
The aim of this research was to study spatial and temporal variation in epiphyte cover and leaf biomass of Posidonia oceanica in Eastern Tunisia. Sampling was conducted at four stations on the Mahdia coast during October and December 2010, and April and August 2011, which correspond respectively to autumn, winter, spring and summer in this area. Posidonia oceanica shoots were collected at two depths (5 and 10 m). Cover of macroinvertebrates and macroalgae was estimated on adult leaves. The results showed that leaf and epiphyte biomasses vary significantly according to sampling date, with the highest values recorded in August. We found a high diversity of epiphytic assemblages on the leaves of P. oceanica with clear qualitative and quantitative dominance of Rhodophyceae compared to other groups, followed by Phaeophyceae. Most epiphyte species on the leaves of P. oceanica in Eastern Tunisia are the same as those in other parts of the Mediterranean Sea. No bathymetrical variation in the epiphytic community was found in our study area, which can be explained by the high levels of water clarity in Mahdia.  相似文献   

8.
《Oceanologica Acta》1999,22(1):109-117
The effects of fish farming on a seagrass (Posidonia oceanica) meadow at Fornells Bay, Minorca (Balearic Islands) were studied. Changes in plant and meadow features (e.g. shoot morphology, shoot density, biomass, rhizome growth, nutrient and soluble sugars concentrations…) in three stations along a transect from a disturbed (organic pollution due to fish cultures) to an undisturbed site were assessed. The fish culture had ceased in 1991; however, seagrass decline, already reported in a previous study for the period 1988–1990, was still taking place at the time of sampling (July 1994). Differences between stations were very clear; the station closest to the fish cages showed reduced shoot density, shoot size, underground biomass, sucrose concentration and photosynthetic capacities. In contrast, shoots from the most polluted station showed higher P-concentration in tissues and higher epiphyte biomass than the other two. Since water conditions had recovered completely by the time of the sampling mission, it is proposed that the persistence of the seagrass decline was due to the excess organic matter remaining in the sediment.  相似文献   

9.
The demand for sensitive biological tools to assess the environmental quality of coastal waters at broad spatial scales is increasing. Many of the tools used are based on the taxonomic composition of biotic assemblages. They usually require a valuable taxonomic expertise while are unique reflecting the overall ecosystem integrity. Here, we evaluate the potential indicator value of several features of the epiphytic community (overall assemblage composition, species richness, and proportion of the main taxonomic groups) growing on the seagrass Posidonia oceanica leaves. We do so by empirically examining their changes along a disturbance gradient where multiple human activities have interactive and cumulative impacts, sampling at different spatial scales and at two different depths (5 and 15 m). Our results show that the specific composition of the epiphytic assemblages (i.e. species composition) closely reflects, in the deep meadows, the combined effects of different anthropogenic stressors along the gradient, showing an integrative and non-specific response. Similarly, an increase in the proportion of hydrozoans, and a decrease in the proportion of rhodophytes and chlorophytes are observed in deep meadows along the gradient. In shallow meadows, grazing and biotic features of the seagrass seem the main forcing factors determining species composition, and therefore masking the response of epiphytes to the deterioration gradient. After address the effect of natural sources of variability (water depth, within- and between-meadow heterogeneity), changes in epiphyte assemblages and in the proportion of hydrozoans, rhodophytes and chlorophytes in relatively deep meadows seem promising monitoring tools for detecting coastal environmental deterioration.  相似文献   

10.
Although seawater desalination has increased significantly over recent decades, little attention has been paid to the impact of the main by-product (hypersaline water: brine) on ecosystems. In the Mediterranean, potentially the most affected ecosystems are meadows of the endemic seagrass Posidonia oceanica. We studied the effect of brine on a shallow P. oceanica meadow exposed to reverse osmosis brine discharge for more than 6 years. P. oceanica proved to be very sensitive to both eutrophication and high salinities derived from the brine discharge. Affected plants showed high epiphyte load and nitrogen content in the leaves, high frequencies of necrosis marks, low total non-structural carbohydrates and low glutamine synthetase activity, compared to control plants. However, there was no indication of extensive decline of the affected meadow. This is probably due to its very shallow situation, which results in high incident radiation as well as fast dilution and dispersion of the brine plume.  相似文献   

11.
Heavy nutrient loads in coastal waters often lead to excessive growth of microalgal and macroalgal epiphytes on seagrass leaves, with varying effects on the underlying seagrasses. This study evaluates the photosynthetic performance, epiphytic biomass and tissue nutrient content of two tropical seagrasses, Cymodocea serrulata and Thalassia hemprichii, in two intertidal areas along the Dar es Salaam coast in the Indian Ocean, a nutrient-rich region at Ocean Road (near the city centre), and a nutrient-poor region at Mjimwema (south of the city centre). Epiphyte biomass was significantly higher at the nutrient-rich site, and epiphytes were associated with reduced photosynthetic performance in both seagrass species at both sites. Likewise, nitrogen and phosphorus tissue content was higher in both species at the nutrient-rich site than at the nutrient-poor site. Epiphytic species composition on the seagrass leaves varied between seagrass species and between sites. Cymodocea serrulata had a higher number of epiphytic species at Mjimwema than at Ocean Road, whereas Thalassia hemprichii had more epiphytic species at Ocean Road than at Mjimwema. Seagrass photosynthetic performance, epiphytic biomass and nutrient content of the seagrasses were shown to be affected by nutrient concentration in the water column. Thus, for the future monitoring of the seagrass meadow, we recommend the use of combined measures such as seagrass performance, epiphytic biomass, nutrient contents and nutrient concentration levels in the water column.  相似文献   

12.
This paper aimed to compare epiphyte assemblages of leaves and rhizomes of Posidonia oceanica exposed to different levels of concentration of nutrients. The same design including a potentially impacted meadow and two reference meadows was used in each of two locations, characterized by the presence of a city or of suspended cages of a fish farm, respectively. This allowed to test for the consistency of responses of epiphytic assemblages to different sources of eutrophication. In both studies, results documented differences in patterns of composition and abundance of epiphytic assemblages on leaves between disturbed and reference meadows, while assemblages on rhizomes did not appear sensitive to this kind of disturbance. Moreover, in potentially impacted meadows, both assemblages showed different patterns of spatial variability compared to reference assemblages. Species composition and abundance of epiphyte assemblages seemed suitable for detecting moderate nutrient increases, even if adequate sampling designs are needed to separate patterns related to the large natural spatial variability of these systems from those related to changes in environmental conditions.  相似文献   

13.
Variability of fish assemblages across habitat structures can depend on spatial scales. A hierarchical sampling design was used to assess the spatial variability of temperate fish assemblages in different habitats and at multiple scales. Underwater visual censuses were carried out along the coasts of Elba Island (NW Mediterranean) on Posidonia oceanica beds, rocky algal reefs and sandy habitat at three spatial scales, namely tens of metres (individual replicates), hundreds of metres (sites) and tens of kilometres (locations). At the assemblage level, there was a clear relationship between fish and habitat type and the observed habitat‐related differences were largely dependent on species identity. Fish assemblages on P. oceanica beds and rocky reefs shared a high number of species, whereas overlap with sandy assemblages was negligible. Multivariate analyses revealed significant differences in fish assemblages among habitats, although there was also a significant habitat × site interaction. These differences relied mainly upon assemblage composition and species richness. Assemblages on rocky reefs and P. oceanica meadows usually harboured a higher number of species and individuals compared with sandy assemblages. Nevertheless, the patterns of habitat‐related differences in species richness and, especially, in the total number of fish, changed significantly from site to site. Eight species showed significant differences over habitats, but they were not consistent due to the interaction of habitat with site. Predictability of fish at both assemblage and population levels decreased with the scale of observation, and the spatial pattern of fish observed at the smallest scale was likely dependent on factors other than habitat type.  相似文献   

14.
The seasonal plasticity of individual Zostera noltii architectural, reproductive and elemental content features, of plant epiphyte load and of meadow biomass–density relationships was investigated along a vertical intertidal gradient at Ria Formosa lagoon, southern Portugal. The vertical variability of the seagrass environment was evident in the sediment characteristics, which showed coarser grain size, less organic matter, lower N content and higher ammonium concentration in the low intertidal than in medium and high intertidal. A clear vertical differentiation in Z. noltii morphology was observed from longer and wider leaves, longer and wider internodes and shorter roots at low intertidal, to shorter and narrow leaves, shorter and narrower internodes and longer roots at high intertidal. The leaf size was negatively related to light availability and positively related to nutrient availability whereas the root size was negatively related to nutrient availability. The lower leaf N and P content found in low intertidal plants may reflect a dilution effect of the nutrients due to higher growth rates. Lower N content of low intertidal leaves supports previous findings that the nitrate reductase activity is lower in plants from this level. The higher epiphyte load observed in Z. noltii leaves of the low intertidal may be a consequence of the lower exposure period, but also of higher hydrodynamics that increase the availability of nutrients. No evidence of the influence of the intertidal level on the flowering shoot density was found. The cyclic temporal pattern of the biomass–density relationship was much wider at low and medium intertidal than at high intertidal. At low intertidal, the decline in shoot density during fall and winter was coincident with a biomass decrease and its increase in spring and summer coincided with the biomass increase. In medium and high intertidal, the biomass and density seasonal variations were decoupled. As a result, only at low intertidal there was a significant positive relationship between biomass and density. This suggests that Z. noltii population structure along the intertidal is regulated by different factors. Light is probably the most important factor regulating the population structure in the low intertidal, whereas desiccation is probably the main factor regulating the populations in upper intertidal. Zostera noltii showed a considerable plasticity at a physiological-, plant- and population-level along the intertidal zone, indicative of the species acclimation to the steep environmental gradient of this particular ecosystem.  相似文献   

15.
The aim of this work is to study the temporal dynamics of rhizome epiphytes and sessile animals living on the rhizomes of the seagrass Posidonia oceanica in the east of Tunisia. Surveys were conducted in October 2009, and in January, April and August 2010 on a fringing reef located in Chebba. Rhizomes were sampled by SCUBA diving at three stations. Samples were examined with a microscope to estimate the cover of macroinvertebrate and macroalgal organisms on the top 10 cm of each rhizome. Results revealed a high diversity of epiphytes on P. oceanica rhizomes with a dominance of red and brown algae, ascidians, and bryozoans. Distinct temporal changes were observed in Oued Lafrann, with a high January cover (winter period) for all groups. These winter increases can be attributed to: (i) the low phenological parameters of P. oceanica in winter that reduce the effects of shading, (ii) life cycles of the epiphytes and invertebrates, (iii) water motion and (iv) grazing.  相似文献   

16.
Although grazing is considered an essential process controlling epiphyte biomass on seagrass leaves, there is still a lack of fundamental knowledge about the species‐specific consumption rates of the most common grazers in Mediterranean meadows. This study experimentally assessed the effect of Posidonia oceanica‐associated gastropod grazing on early successional biofilm and the species‐specific relationship between biofilm consumption rates and biofilm biomass. Two biofilms on artificial substrata, both developed in situ (in a P. oceanica meadow), one under ambient conditions and the other under nutrient‐enriched conditions, were offered in aquaria assays to nine species of grazers found in P. oceanica meadows. Biofilm consumption rates and their association with biofilm biomass were assessed. It was found that: (i) there was a positive association between biofilm consumption and biofilm biomass up to 20 mg Chl a·m?2 for Bittium reticulatum, Gibbula ardens, Jujubinus exasperatus and Tricolia pullus; (ii) Alvania montagui, B. reticulatum and Jujubinus striatus showed the highest consumption rates and are thus expected to be amongst the leading consumers in early‐successional epiphytic communities; (iii) there was not an increase of consumption rate when a substratum colonized under nutrient‐enriched conditions was offered to any of the nine studied species. This study provides species‐specific consumption rates knowledge that is useful for the assessment of the strength of grazer–epiphyte interactions and trophic fluxes in P. oceanica meadows.  相似文献   

17.
The meadows of the Mediterranean endemic seagrass Posidonia oceanica exhibit relatively high variations of structural and biometric features at various spatial scales. An investigation performed in 1992 in the meadow off Lacco Ameno (Island of Ischia, Gulf of Naples, Italy) detected peculiar spatial patterns of plant distribution, characterized by nestlike structures with radially increasing or decreasing shoot densities. Eight years later (2000), geo‐referenced collections at selected points were repeated to trace the temporal variations of shoot density and investigate the recurrence of the density cores previously detected. In addition, shoots for molecular analyses were collected to check the hypothesis that nestlike patterns exhibit highest levels of genetic variability, due to the confluence of several genetically distinct stolons. The 2000 survey confirmed the presence of the main density cores detected in 1992, although their spatial distribution was slightly shifted and a general decrease of spatial anisotropy was observed, probably due to an increased disturbance, mainly due to pleasure boat anchoring. Patterns of genetic diversity showed a more complex picture, well related to the shoot density spatial pattern, especially when compared with the previous 1992 survey. Patterns of genetic diversity confirmed our previous hypotheses on the genesis of shoot density cores, suggesting they are produced over long time, due to a slow stolonization process and a convergence of different genotypes. Regression of the meadow and decrease of density may lead, in short periods, to a homogenization of the density patterns, while genetic diversity cores represent a long‐term ‘memory’ of their previous distribution.  相似文献   

18.
Abstract. The temporal dynamics of three seagrasses, Posidonia oceanica, Cymodocea nodosa and Zostera marina, was studied in different areas of the Adriatic Sea by analysing phenological parameters and biomass trends in different compartments of seagrass systems. For this purpose, samplings were conducted in 1997 once per season at each station, Otranto (southern Adriatic Sea) and Grado (northern Adriatic Sea). Structural parameters and biomass of plant compartments differed among seagrasses both in absolute values and in seasonal variability. P. oceanica was the largest plant, showing the highest number of leaves per shoot, highest leaf surface, Leaf Area Index and shoot weight. Z. marina was intermediate in size and had the longest leaves, whereas C. nodosa was the smallest seagrass. P. oceanica accounted for the highest total biomass (mean ± SE: 1895.9 ± 180.2 g DW · m–2; CV = coefficient of variation: 19.0 %), considerably more than C. nodosa (mean ± SE: 410.4 ± 88.4 g DW·m–2; CV: 43.1 %) and Z. marina (mean ± SE: 312.1 ± 75.1 g DW · m–2; CV: 48.1 %), although the two latter species displayed a higher seasonal variability. Similarly, other features, such as shoot density, leaf surface, LAI, shoot weight and relative contributions of above‐ and below‐ground compartments, were less variable across seasons in P. oceanica than in the two other seagrasses, while leaf length showed the highest seasonal fluctuation in P. oceanica. As for biomass partitioning, C. nodosa showed a higher proportion of the below‐ground relative to above‐ground biomass (up to 90 %), with a distinct seasonality, whereas in P. oceanica the proportion of below‐ground biomass (around 80 %) was fairly constant during the year. We infer that in P. oceanica the seasonal forcing is probably buffered by the availability of internal resources stored permanently during the year in the below‐ground. In C. nodosa and Z. marina, instead, growth processes seem to be amplified by a greater influence of environmental factors.  相似文献   

19.
A significant consideration in defining patterns of ecological interest is determining at what spatial scales these patterns are apparent. We performed a mensurative experiment to determine the effect of how changing the extent of sampling, using a fixed grain (panning up), changed our measurements of seagrass epiphyte biomass at a variety of hierarchical spatial scales, ranging from approximately 0.1 m to 2.5 × 105 m. The experiment was performed twice at two sets of nested locations, also capturing an element of seasonality within the spatial extents. While the two regions and their nested locations differed in terms of epiphyte biomass, and values did change over time, mean epiphyte values were invariant at the local scale. This indicates that at the local scale (≤100 m extent), we expect seagrass epiphyte biomass to be relatively homogenous. Contrary to theoretical predictions, we did not detect predictable changes in mean values of epiphyte biomass as sampling extent increased. The results of this study highlight the importance of recognizing the spatial dependence of patterns in heterogeneous environments.  相似文献   

20.
Gut content examination and trophic markers (fatty acids, stable isotopes of C and N) were combined to delineate the diet of the dominant species of amphipods from Mediterranean Posidonia oceanica seagrass meadows and to highlight trophic diversity among this community. Our results indicate that, although all dominant species heavily relied on macroalgal epiphytes, considerable interspecific dietary differences existed. Carbon stable isotope ratios notably showed that some of the amphipod species favored grazing on epiphytes from leaves or litter fragments (Apherusa chiereghinii, Aora spinicornis, Gammarus aequicauda), while others such as Dexamine spiniventris preferred epiphytes from rhizomes. The remaining amphipods (Caprella acanthifera, Ampithoe helleri and Gammarella fucicola) readily consumed both groups. In addition, SIAR modeling suggested that most species had a mixed diet, and relied on several food items. Fatty acid analysis and gut contents revealed that contributions of microepiphytic diatoms and of benthic and suspended particulate organic matter to the diet of amphipods were anecdotal. None of the examined species seemed to graze on their seagrass host [low 18:2(n‐6) and 18:3(n‐3) fatty acids contents], but Gammarus aequicauda partly relied on seagrass leaf detritus, as demonstrated by the lesser 13C‐depletion of their tissues. Overall, our findings suggest that amphipods, because of their importance in the transfer of organic matter from primary producers and detritus to higher rank consumers, are key items in P. oceanica‐associated food webs.  相似文献   

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