بررسی تغییرات رژیم غذایی پاروپایان با استفاده از پروفایل اسیدهای چرب در تنگه هرمز

The present study was conducted with the aim of investigating variations of diet of copepods in the Strait of Hormuz in both cold and warm seasons using fatty acid profiles with considering the effect of dominant phytoplankton groups in the region. The studied stations were selected as a transect perpendicular to the Bandar Abbas coast and in the center of the Strait of Hormuz. Four stations were sampled in this transect. Sampling was done in two hot seasons (July) and cold season (January). A vertical plankton net with an opening of 50 cm, a length of 2 m, and mesh size of 300 μm in the body part and 100 μm in the collecting container (Netbucket) was connected to a winch from the upper part and a 5 kg weight from the end part. Sampling was done from a depth of 10 meters to the water surface as a vertical pull between 9:00 and 15:00. This operation was repeated three times in each station. The first and second replications were fixed using 4% formalin and transferred to 500 ml containers, and the third repetition was placed in a -40°C freezer located on the research vessel and transferred frozen to the laboratory. The first and second replicates were used to identify and enumerate copepods, and the third replicate was used to isolate and prepare fatty acid profiles. To measure environmental parameters, water samples of each layer (5 meters and 15 meters) were transferred to the floating surface using a Niskin bottle, and parameters were measured using a Hach multimeter. In order to identify phytoplankton groups, one liter of water was taken from the water surface at the sampling point and poured into dark containers and fixed with acidic Lugol solution and stored in cool conditions at room temperature (below 25°C).
To concentrate phytoplankton cells, the method of sedimentation process was repeated twice. In this method, the water sample was transferred to a one-liter graduated cylinder and after 72 hours, 90% of the surface water was decanted. Then, the remaining sample volume was transferred to a 100 ml graduated cylinder and again after 72 hours, 90% of the surface water was decanted. After that, to remove the concentrated sample, the container was stirred to mix well, then using a pipette, one milliliter of the sample was transferred to the Sedwig Rafter counting slide. Samples were analyzed using a Nikon inverted microscope with 400x magnification. Finally, the number of cells of each phytoplankton group per milliliter was calculated.
Zooplankton samples were separated from the fixative solution using a 100-micron sieve. Then the samples were transferred to 70% alcohol and glycerin solution. After that, the copepod samples were separated from other zooplankton using a Leica stereomicroscope. Finally, different copepod species were identified at the genus level. Using numbers recorded by the flowmeter, the volumes of water filtered by the plankton net was calculated for each net haul. Finally, the density of copepods was calculated as number per cubic meter. The separation of copepods from frozen zooplankton samples was done quickly and under conditions of near zero temperature. Then the isolated copeod samples were frozen again. Finally, the samples were transferred to the central laboratory of the National Research Institute of Oceanography and Atmospheric Sciences in frozen form for analyzing fatty acids content.
Cyanobacteria with 52% relative abundance were the dominant group in July. All the cyanobacteria observed in this study were Trichodesmium erythraeum, and after those, diatoms and dinoflagellates formed 36 and 12% of the phytoplankton groups in July, respectively. In January, diatoms were the most abundant phytoplankton group (74%) and cyanobacteria were rarely found (1%). A total of 17 genera of copepods were identified. Acartia sp. (30.57 %) and Oithona sp. (19.03 percent) the most abundant copepods were in July. In January, Calanopia sp. (16.2 percent) and Oncaea sp. (15.26%) were the most abundant copepods. The feeding type of copepods changes according to the season and available prey. In this study the majority of observed copepods were herbivores and omnivores.
In July and January, hexadecanoic fatty acid has the largest share of the identified fatty acids in the total of fatty acids with 32 and 27%, respectively. The cis-Vaccenic index fatty acid also accounted for 9 and 4% of the total identified fatty acids in July and January. Docosahexaenoic fatty acid (DHA) with 4% in July and eicosapentaenoic fatty acid (EPA) with 2% abundance were found in January samples. In July, 4% of DHA was found in the total amount of fatty acids, which indicates the consumption of flagellates by copepods. Higher abundance of flagellates in July and abundance of Oithona sp. which prefer the consumption of flagellates caused Higher DHA percentage in summer samples. In January, EPA constituted 2% of the total amount of fatty acids, which indicates the consumption of diatoms by copepods in winter.
The results of this study showed that the zooplankton community structure is significantly different between the cold and warm seasons in the Strait of Hormuz. Dominant phytoplankton groups also change significantly in the Strait of Hormuz in both cold and warm seasons, and the effect of changes in the community of primary producers in the Strait of Hormoz can be observed by tracking fatty acid markers in secondary producers. The environmental conditions in summer and winter are significantly different in the Strait of Hormuz. Water temperature and salinity were higher in July and the amount of dissolved oxygen measured in January was lower. However, we cannot obtain detailed information about the nutritional relationship of copepods and phytoplankton groups with the results of this study, but we can increase our general knowledge about their nutritional relationships in the Strait of Hormuz.
Obviously, the amount of production in summer was higher than in winter. But the composition of prey available to primary consumers (herbivorous copepods) has changed. In this way, in summer, cyanobacterial flagellates were more available than diatoms. The question is, do copepods directly consume cyanobacteria and lead to continuation of the bacterial production in the food chain? By comparing the relative frequency of cis-Vaccenic acid as an indicator of bacteria in winter and summer, it can be said that the production by bacteria is transferred to the primary consumers, but the direct consumption of cyanobacteria by copepods cannot be commented. Whereas, harpactoid species known as direct consumers of cyanobacteria were not observed in this study. Regarding flagellates and diatoms, it can be said that the flagellates in summer and diatoms in winter has a greater contribution to the diet of copepods in the Strait of Hormuz.