Choices Of Floating Raft Macroalgae By Two Amphipods Species

Mussel farming is the main method of shallow sea raft farming in China.The complex three-dimensional habitat formed by rafts provides habitat and shelter for many marine organisms,especially those epiphytic macroalgae,small invertebrates and juvenile fish.Amphipoda organisms such as Gammaridea and Caprellidae often onmacroalgae for evasion,feeding and other life activities.The secondary production of amphipods in the mussel culture area is a key link of energy flow and logistics in the marine ecosystem.The study of the population characteristics and energy sources of amphipods is an important step to explore the adaptation mechanism of amphipods to the environment,which is helpful to further understand the nutritional relationship between species in marine ecosystems.In this stydy,by collecting macroalgae and amphipod attached to the rafts of the mussel culture area,combined with the stable isotope of carbon and nitrogen,we analyzed the community structure and spatiotemporal variation characteristics of floating raft macroalgae,Caprellidae and Gammaridea population characteristics,the effects of floating raft macroalgae on amphipods population characteristics and the feeding ecology of amphipods.The main research conclusions are as follows:?1?A total of 49 species of epiphytic macroalgae were collected in four seasons throughout the year,Jania decussato-dichotoma and Hypnea boergesenii is the dominant species throughout the year on rafts;seasonal variation of the number of species:autumn?29 species?>summer?22 species?>winter?21 species?>spring?20species?;seasonal variation of the average biomass:summer?5529.395g/m²?>spring?3508.583g/m²?>winter?2441.807g/m²?>autumn?2340.446g/m²?;the horizontal distribution of species number and biomass are gradually decreasing from the inside to outside of the culture zones;the seasonal variation of epiphytic macroalgae diversity index is autumn?2.3?>spring?2.1?>winter?2.0?>summer?1.87?;among the dominant macroalgae,the maximum and minimum niche breadths appear in autumn,which are Hypnea boergesenii?2.214?and Sargassum horneri?0.739?;the maximum and minimum values of niche overlap occur in autumn,which are Sargassum horueri-Gracilaria textorii?0.861?and Grateloupia turuturu-Sargassum horueri?0?.Although the number of species and biomass of macroalgae on buoyant in the culture area is lower than that of natural macroalgae,its scale far exceeds that of benthic algae.It has formed a relatively stable algae community and interacts with cultured mussels and other marine organisms,and form an artificial habitat that approaches the ecological function of natural algae farms.?2?The population density of Caprellidae and Gammaridea showed a seasonal change pattern in summer?56.23±34.24 ten thousand/m²and 13.20±10.02 ten thousand/m²?>spring?7.91±6.70 ten thousand/m²and 2.45±2.24 ten thousand/m²?>autumn?0.44±0.30 ten thousand/m²and 0.72±0.50 ten thousand/m²?>winter?0.02±0.03 ten thousand/m²and 0.44±0.28 ten thousand/m²?.Caprellidae are mainly distributed in the middle and outside of the culture area,and Gammaridea are mainly distributed in the inner side of the culture area near the wharf and the middle of the culture area.The results of RDA ordination showed that the main environmental factors affecting the population density of Amphipoda in spring,summer,autumn and winter were NO₃^-?32.8%?,temperature?43.7%?,NO₂^-?30.3%?and PO4₃^-?40.4%?.In the four seasons of spring,summer,autumn and winter,the maximum density of Caprellidae was found on the Chondria dasyphylla?50 ind/g?,Hypnea boergesenii?212 ind/g?,Ganonema farinose?97 ind/g?and Chondria dasyphylla?2 ind/g?,and the minimum density was found on Ganonema farinose?16 ind/g?,Ulva pertusa?73 ind/g?,Ulva pertusa?1 ind/g?and Jania decussato-dichotoma/Ulva pertusa/Ganonema farinose?0ind/g?,the maximum density of Gammaridea was Pachydictyon coriaceum?27 ind/g?,Pachydictyon coriaceum?46 ind/g?,Jania decussato-dichotoma?15 ind/g?,Ganonema farinose?73 ind/g?,and the minimum density was Ulva pertusa?4 ind/g?,Chondria dasyphylla?13 ind/g?,Ulva pertusa?2 ind/g?and Jania decussato-dichotoma/Hypnea boergesenii?3 ind/g?,respectively.The adhesion rule of Caprellidae on different forms of macroalgae is filamentous?65±100 ind/g?>clustered?50±56 ind/g?>leafy?36±52 ind/g?>flaky?23±34 ind/g?.The adhesion rule of Gammaridea on different forms of seaweeds is leafy?22±19 ind/g?>flaky?19±19 ind/g?>clustered?18±8 ind/g?>filamentous?13±10 ind/g?.The seasonal variation and distribution of population density of two species of amphipoda are affected by many environmental factors.Caprellidae and Gammaridea have certain selectivity to inhabit algae,preferring to live on the floating raft algae with large biomass and complex morphology.?3?The?¹³C and?¹⁵N values of amphipoda are between-20.42‰?-14.72‰and0.95‰?4.52‰,and the potential food sources of amphipoda include macroalgae and epiphytes,phytoplankton and zooplankton,their?¹³C and?¹⁵N values range from-20.71‰to-10.78‰and 0.41‰to 7.50‰,respectively.In summer,the main carbon sources of Caprellidae that onthe dominant macroalgae species?Gracilaria textorii,Pachydictyon coriaceum,Chaetomorpha aerea,Ganonema farinose?were macroalgae and their epiphytes?contribution rates are 46.4%,49.3%,46.8%and 38.4%?,the main food source of the Caprellidae that lived on Ulva lactuca was zooplankton?contribution rate was 49.2%?;the main carbon sources of Gammaridea that onthe five dominant macroalgae species were macroalgae and their epiphytes?contribution rates are 45.9%,41.1%,52.4%,72.8%and 57.2%?.In winter,the main food sources of Gammridea on the dominant macroalgae species??were epiphytes,and their contribution rate exceeds70%.The average contribution rate of macroalgae,POM,phytoplankton and zooplankton to the carbon source of Gammaridea is small.The feeding of Caprellidae on different macroalgae is related to the type of seaweed,and the feeding of Gammaridea is relatively simple,and the selectivity for inhabiting macroalgae mainly depends on the morphological structure of macroalgae.