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Lab Protocol

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GEOPHYSICS

TAMAR SAMPLING

OFFSHORE

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Home Geophysics Tamar Sampling Offshore

BIOLOGICAL

PHYTOPLANKTON & ZOOPLANKTON ANALYSIS

Throughout the transect, zooplankton community composition varies, with some types not being present at various depths and stations. However, the one consistency that remains throughout is the large amount of copepoda. These tiny crustacea continued to remain the dominant zooplankton throughout the transect, portraying their adaptability to a range of abiotic and biotic factors, regardless of well-mixed or stratified waters. The data here was log transformed on account of the heavy influence of copepods, which occurred in numbers several magnitudes of volume higher than other zooplankton. The highest abundance of zooplankton occurs at site 37. There doesn’t appear to be a significant trend between site and total abundance. Likewise, there appears to be no correlation between depth and total abundance. With only 5 samples however, it is difficult to draw any informed conclusions. To better assess the effect of depth on zooplankton distribution, a continuous plankton recorder could be used. This would allow us to sample larger areas in smaller depth ranges.

Diversity

The pie charts for deepest waters show a dominance by copepods. This is likely due to their variety of feeding types and mobility, allowing them to cover large areas, increasing their prey encounter rate and consume falling detritus from the more productive surface waters whereas other species such as Noctiluca only appear to be present in the upper 15m of water around high phytoplankton abundance, suggesting they are less mobile and rely on larger blooms to sustain them. Site 35 is the most diverse, however all sites maintain a level of diversity largely equivalent to each other. The shallower depths have a large amount of sunlight, allowing for phytoplankton bloom, a useful resource that can be utilized by this large variety of species. All in all, it appears as though greater diversity correlates with shallower depths. Many of the species observed in these areas are in their larval stages. The copepod nauplii as an example, are present in the highest abundance in the upper 20m. It is also important to note that some species appear early on in the transect, but stop appearing at later sites. Types of plankton that fall into this category include Noctiluca and decapoda larvae, perhaps indicating their inability to survive in more stratified waters.

Much like zooplankton abundance, species composition of phytoplankton varies throughout different depths and sites along the offshore transect. The species found to have the highest abundance overall was Ceratium lineatum (figure phytoplankton pie chart) but unlike copepoda (figure zooplankton), this consistency is not constant with every site, such as the species only being dominant at two depths at site 35, but not being seen at 29m, perhaps caused by the lower light attenuation. This pattern continues, whereby when this species is present, it is found to dominate, but only at shallow depths. Some mid depths of around 15-30m show dominance by Chaetoceros. However, this species is absent from site 35 at the equivalent depth. Instead at this depth, Ceratium furca dominates. The highest total abundance of phytoplankton can be found at 16.9m deep at site 40. This corresponds to where we found the strongest evidence of a thermocline, supporting the idea of a deep chlorophyll maximum occurring where the upper light rich surface waters meet and mix with the lower nutrient rich waters. At this depth we observed large amounts of Mesodinium, a highly mobile ciliate which thrives in low nutrient surface waters by actively moving between shallow and deeper waters, gathering nutrients from depth and photosynthesizing in surface waters (Lips and Lips 2017). Aside from site 40, the highest levels of chlorophyll are found in surface waters, suggesting that in these well mixed waters, light availability is the main factor influencing bloom dynamics.

Diversity

Depths with highest species diversity are those that are shallow. In the shallow waters at site 40 however, large amounts of Dactyliosolen fragilissimus are present in isolation to any other species. This species is known to occupy stratified surface waters (Lotocka 2006) and is absent from any other sites. This perhaps is evidence of the formation of the thermocline and the corresponding temperature and nutrient mixing changes leading to completely different bloom formations. Another exception occurs at a depth of 29m, having 4 different species present, and a substantial number of each, whereas site 37, at a depth of only 1.3m more, only 3 species were present, and were found to be fewer in abundance. This would most likely imply that again, a differing amount of stratification is causing a change in nutrient distribution in the water column, leading to slower growth rates and a loss of diversity.

DCM

Subsequent to spring blooms as irradiance increases and nutrients become depleted at the surface a deep chlorophyll maximum forms, as observed at station 40, furthest offshore, at 16.9m. The deep chlorophyll maximum (DCM) is a subsurface layer enriched in chlorophyll (Chl), that typically develops coinciding with the thermocline/nutricline enabling phytoplankton to meet their growth requirements from cool, nutrient rich deep waters below and high irradiance surface waters above (Latasa et al.). Here diatoms are likely to be the most successful phytoplankton group, such as Chaetoceros at station 40 due to adaptations allowing them to thrive in productive regions including high growth rates, low Kn values and grazer defence mechanisms (Widdicombe et al.) (Merzouk et al.).

DAPI MICROSCOPY AND IMAGE ANALYSIS

META DATA:

DATE: (09/07/19)

STATION 40: (50° 05.953, 004° 14.918)

FILTER 4 (43.2m)

FILTER 10 (7.7m)

FILTER 14 (16.9m)

Having DAPI-stained micro-organisms from station 40 samples, furthest offshore (50° 05.953, 004° 14.918) onto three filters the cells were visualised using epifluorescence microscopy and image analysis which in turn aided in obtaining total microbial cell abundances and photosynthetic abundances in the samples.

The epifluorescence microscope (courtesy of the Marine Biological Association), excited fluorophores in the sample at approximately 475nm using a dichroic mirror. In response they emitted light at a specific wavelength which was longer and lower energy which allows individual fluorophores, in this case chlorophyll, in the sample to be detected. Using a DAPI (which binds to DNA) overlay – we can assess which of these fluorophore emissions are representative of true phytoplankton cells in the sample. If there is no overlap, this provides potential evidence of heterotrophic cells or cyanobacterium in the sample that absorb different wavelengths of light. Five sets of images (DAPI and autofluorescent) were taken for each of the three filters and loaded into ImageJ software in which mean cell counts were conducted and total cell counts were obtained via the following equations.

A total microbial biomass was then obtained by multiplying the mean cell abundance by 5Cml-1-assuming that this is the lowest cell-specific carbon content (Whitman et.al, 1998).

The graph illustrating the estimated microbial biomass demonstrates that phytoplankton cells, represented via chlorophyll biomass, were most abundant at 16.9m (398.148 fg C ml-1) at station 40. This would be expected due to the respective requirements of phytoplankton cells for higher irradiances are corresponds with the  location of the DCM as shown via CTD profiles. Consequently, chlorophyll biomass was extremely lower at 43.2m (Alternatively, the highest DAPI biomass is located at 43.2m (1189.3625 fg C ml-1), perhaps due to a lower requirement of heterotrophic bacteria and cyanobacteria for high irradiances. It is also plausible that the heterotrophic bacteria locate at the base of phytoplankton activity to obtain raining dissolved organic matter via marine snow and phytoplankton detritus (Branco et.al, 1996).

At station 40, three samples were taken to undergo flow cytometry in the lab. Cyto 82 was taken at 17.7m depth, Cyto 89 at 16.9m and Cyto 87 at 4.6m. Cytometry is used to count both the number of cells in a sample, as well as the red fluorescence seen, which can be used to indicate chlorophyll levels. The Cyto 82 and Cyto 89 represent samples which were taken either side of the chlorophyll maximum, as seen in the depth profile for station 40.

What can be seen from the results in figures 73 and 74 is that for all depths, the nanoplankton have a much higher value for both the cell numbers and the red fluorescence. As well as this, 16.9m has the highest values for all nanoplankton and total counts, yet for microplankton both higher values occur at 17.7m