Seagrass: A Dynamic System

<p>Small herbivorous invertebrates consume algal epiphytes, reducing negative effects (e.g. shading) on seagrass. Much research to date has focused on crustacean grazers, and comparatively little on gastropods. The aim of this research was to 1) examine seasonal and spatial variation in seagrass and associated gastropods in a large, tidal estuary and 2) examine the response of seagrass and epiphytic algae to nutrient enrichment and grazing pressure in a laboratory experiment. Surveys were conducted in summer and winter of 2016 to assess the seasonal fluctuations in the associated gastropods and relationships with seagrass and epiphyte biomass within three sites in the Porirua Harbour. Seagrass, gastropods and epiphytic algae showed seasonal trends, including evidence of a loss of grazer control on epiphytes during winter. Potamopyrgus estuarinus, Notoacmea scapha, Diloma spp. and Micrelenchus spp. were the dominant gastropod grazers in the system in both seasons. The gastropod assemblage and seagrass characteristics differed between sites, likely in response to small scale differences in abiotic factors. Seagrass from Elsdon (a site with elevated nutrient levels) and Browns Bay (a relatively pristine site) were used to investigate the role of select grazers and nutrient enrichment on epiphyte and seagrass growth. Nutrient treatments represented nitrate and phosphate concentrations of Elsdon (High), a 20% increase (High+) and a control (no addition). Little evidence was found for epiphyte regulation by gastropods, nor did epiphyte loads increase with nutrient addition. Seagrass from Browns Bay responded more strongly to High+ treatments than that of Elsdon. The results suggest that seagrass from Elsdon is adapted to the site’s high nutrient loads, where seagrass from Browns Bay is not. The results of this thesis support prior research findings of high variation in seagrass over a small scale, and adds to the currently lacking information on the role of micro-grazers in New Zealand’s seagrass meadows.</p>

Seagrass: A Dynamic System

<p>Small herbivorous invertebrates consume algal epiphytes, reducing negative effects (e.g. shading) on seagrass. Much research to date has focused on crustacean grazers, and comparatively little on gastropods. The aim of this research was to 1) examine seasonal and spatial variation in seagrass and associated gastropods in a large, tidal estuary and 2) examine the response of seagrass and epiphytic algae to nutrient enrichment and grazing pressure in a laboratory experiment. Surveys were conducted in summer and winter of 2016 to assess the seasonal fluctuations in the associated gastropods and relationships with seagrass and epiphyte biomass within three sites in the Porirua Harbour. Seagrass, gastropods and epiphytic algae showed seasonal trends, including evidence of a loss of grazer control on epiphytes during winter. Potamopyrgus estuarinus, Notoacmea scapha, Diloma spp. and Micrelenchus spp. were the dominant gastropod grazers in the system in both seasons. The gastropod assemblage and seagrass characteristics differed between sites, likely in response to small scale differences in abiotic factors. Seagrass from Elsdon (a site with elevated nutrient levels) and Browns Bay (a relatively pristine site) were used to investigate the role of select grazers and nutrient enrichment on epiphyte and seagrass growth. Nutrient treatments represented nitrate and phosphate concentrations of Elsdon (High), a 20% increase (High+) and a control (no addition). Little evidence was found for epiphyte regulation by gastropods, nor did epiphyte loads increase with nutrient addition. Seagrass from Browns Bay responded more strongly to High+ treatments than that of Elsdon. The results suggest that seagrass from Elsdon is adapted to the site’s high nutrient loads, where seagrass from Browns Bay is not. The results of this thesis support prior research findings of high variation in seagrass over a small scale, and adds to the currently lacking information on the role of micro-grazers in New Zealand’s seagrass meadows.</p>

Snail communities improve submerged macrophytes growth by graze epiphytic algae and phytoplankton in a mesocosm experiment

The relationship between producers (e.g., macrophyte, phytoplankton and epiphytic algae) and snails plays an important role in maintaining the function and stability of the shallow ecosystems. A complex relationship exists among macrophytes, epiphytic algae, phytoplankton and snails. An outdoor mesocosm experiment with two-way factorials was carried out, three species submerged macrophytes (Hydrilla verticillate, Vallisneria natans or one exotic submerged plant Elodea nuttallii) and two grazing treatments (4 snail species present or absent) to elucidate those relationships. The results showed that the snail communities reducing the biomass of phytoplankton and epiphytic algae indirect then enhanced the growth of the submerged macrophytes. The macrophyte with complex architecture supported more snail and epiphytic algae, and snails preferred to feed on native plants. Competition drove snails change the grazing preferences to achieve coexistence, so that led to the assembling of snail communities towards the direction of highest resource utilization.

Epiphyton of oligotrophic lakes of Leningrad region

Here are results of investigation of taxonomic composition, community structure and dominant species of epiphytic algae in 12 oligotrophic lakes located in different geomorphic regions of Leningrad region. In summer epiphytic communities there were 385 algal taxa, they belonged to 6 taxonomic divisions: Cyanophyta (Cyanoprokaryota) – 50 (13%), Bacillaryophyta – 175 (45%), Chlorophyta – 37 (10%), Charophyta – 118 (30%), Xanthophyta – 4 (1,55%), Rhodophyta – 1 (0,45%). The taxonomic structure of each lake was predominant diatoms and desmids, but the biomass was dominated by zygnems and green algae. In total, the taxonomic composition of the epiphyton in the studied lakes is similar, the differences are revealed at the species level. The structure of dominanting epiphyton complex was rather constant and composed by a small number of species. The greatest contribution to the epiphyton was made by Tabellaria flocculosa and species of the genus Bulbochaete. The structure of the epiphyton reflects the undisturbed nature of lake ecosystems, as well as their northern location.

ESTIMATION THE FACTORS AFFECTING ON GROWTH OF ALGAE IN UM EL-NAAJ LAKE BY USING REMOTE SENSING TECHNIQUES

Epiphytic algae adherent to aquatic plants are an essential link in the composition of the food chain of any ecosystem. Epiphytic algae act as primary producers of the food chain in the aquatic ecosystem and as natural food for herbivorous zooplankton and fish. This study aimed to detect the presence of algae colonies through remote sensing and to analyze factors that affect the growth of algae through field survey and visual interpretation of satellite images in Lake Um El-Naaj. Samples were collected from six locations on Lake Um El-Naaj from November 2018 to June 2019. The algae samples were collected from submerged parts of emerging aquatic plants (aquatic macrophytes Phragmites australis) and stored in plastic bags with little ambient water and solutions for preservation in the field and the laboratory. Water samples were collected to study the nutritional factors that influence the growth of algae, including the concentration of Phosphate, Nitrate, and Silica (silicon dioxide). The results showed that the values of total phosphorus increased by 1.0, 0.9, and 0.8 mg/L, in January, in places 2, 5, and 6, respectively. The highest nitrate value was 11.2 mg/L in December at site 5, while the lowest concentration values were in November (2.2 mg/L at site 6 and 3.7 mg/L at site 4). Besides, the lowest silicate concentration was 0.4 mg/L in November at site 2, while the highest was 2.4 mg/L in June 2019 at site 6. Based on the findings, it is possible to conclude that, during the winter, the water level increased due to the rain. For this reason, nutrient concentrations were low during the last period. Also, with the use of maps and remote sensing techniques, it is possible to determine the expected values around the station's location as future predictive readings that compensate for the difficulty of reaching these areas.

Brachyuran Crabs (Decapoda) Associated with Rhodolith Beds: Spatio-Temporal Variability at Gran Canaria Island

Crustaceans are a key component of the fauna living in rhodoliths, but patterns in their distribution and abundance remain largely unknown. This paper assessed spatio-temporal variability of Brachyura associated with rhodoliths. A seasonal study was conducted at three depth layers (18, 25, and 40 m), throughout two years (December 2015 to October 2017) at Gran Canaria Island (eastern Atlantic Ocean). A total of 765 crabs belonging to 10 species were collected. A larger abundance and richness of crabs at 25 m correlated with a larger biomass of epiphytic algae attached to rhodoliths. A seasonal pattern was also observed, where a higher richness of crabs occurred in the summer. The Xanthid crab, Nanocassiope melanodactylus, dominated the assemblage (83%); juveniles of this species were more abundant in deeper waters (40 m), while adults were more abundant on the shallower depth layers (18 m and 25 m). The species Pilmunus hirtellus was restricted to 25 m. Nevertheless, Pisa carinimana and Achaeus cranchii did not show any spatio-temporal pattern. In summary, this study demonstrated that two conspicuous crabs, N. melanodactylus and P. hirtellus, associated with rhodolith beds are bathymetrically segregated.