Falmouth Group 8

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Phytoplankton

At stations 1, 2, 5 and 7, diatoms dominated the sample bottle, comprising over 70% of the sample. At stations 3, 4 and 6, around 80% of each sample was dominated by dinoflagellates.

The figure showing the abundance and diversity of phytoplankton with depth reveals that the two variables are positively related to each other, with a peak occurring at approximately 20 m. The values for abundance and diversity decrease to surface values by 40 m. The trends in both variables appear to be most closely linked below 20 m.

The samples were taken offshore at the beginning of July, signifying the beginning of the summer bloom period for phytoplankton. As a result some areas were still dominated by diatoms whilst other areas were experiencing the changeover to summer dinoflagellate dominance. It is likely that as the season progresses dinoflagellates will be dominating more extensive areas offshore from the coast.

In contrast to the estuary results, diversity was positively linked to abundance. This may be due to the fact that out at sea, the salinity range is optimum for most marine phytoplankton. As a result of this no single phytoplankton group outcompeted the others with regards to salinity adaptations, allowing more groups to co-exist with each other. The peak in abundance and diversity at 20 m links to the thermocline depth measured from many of the stations, where phytoplankton are known to gather.



Zooplankton

It is clear from the figure of zooplankton that the sample area is dominated by copepods, which make up an average 47.4% of each sample recovered. The dominance of this group is seen to its greatest extent at site 2 (25-15m) and site 7 (15-0m), and is lowest at site 4 (20-0m). At this site, polychaete larvae dominate, and this domination continues through station 5. Several other groupings are also notable within the samples recovered. Copepod nauplii   feature in every sample collected, and although never dominant, average at just over 10% abundance per sample. Many of these bottles feature large numbers of cladocera, and in several cases these are the most abundant grouping after the copepoda. The only exception to this is found in bottle 1B (15-0m) and bottle 6B (15-0m), where no cladocera are recorded. Other species found feature in minor amounts, and any minor peaks are not large enough to note.
During the collection of geophysical data the area sampled was filmed using a video sled unit, and the footage from this raised questions suggesting further testing. The seabed in this area is covered in dense layers of Laminaria spp., and hides many rocky ledges, providing protection from the effects of the tide and currents. It was hypothesised that this area may be utilised by certain species as a ‘nursery ground’ for developing young and a zooplankton vertical trawl net three days later provided these results. It was initially expected that copepod nauplii would dominate this area, but the obtained results show a later reproductive stage than previously expected. Previous research indicated up to an 80% dominance by copepod nauplii in the June period, but the results seen here indicate an earlier breeding period, with many of the young maturing into adult forms by this time. As significant alteration in reproductive cycles are usually indicative of major oceanic or estuarine changes this pattern would be worth investigating more closely in the future.

Due to similar sampling ranges being used across different stations several of these data points are replicates of a certain depth. This time there is no clear link between abundance and diversity with depth. Where several replicates are plotted a considerable range has resulted, particularly at 7.5 m depth.

There is no clear trend with zooplankton abundance and diversity with depth. It could be due to zooplankton not being as dependent on local abiotic conditions as phytoplankton, which are primary producers. In addition it is notable that relatively fewer larvae groups were present at these offshore stations. Given the role of estuaries as nurseries for various marine biota this could explain the greater prevalence of larvae groups further inshore. The available data is insufficient to examine the differences in abiotic conditions between stations in great detail, but if it had been it could explain the considerable ranges resulting at specific depths. It could also potentially explain the factors driving zooplankton abundances and diversity in the water column.



Plankton Offshore Falmouth

Click graphs to enlarge

O2 saturation increases from 103% at surface to 115% at 20m depth, this correlates with the chlorophyll concentration, which increases from 2.4 µg/L at the surface to 5.2 µg/L at 20m. Chlorophyll exists in photosynthesising organisms, which release O2 as they photosynthesise. Therefore, it is safe to assume that the peak in O2­ at 20m depth is due to the increases concentration in chlorophyll, and therefore phytoplankton. Below 20m, both the chlorophyll concentration and dissolved O2 concentration decrease to 1.8 µg/L and 97% respectively, before rising again to 4.6 µg/L and 113% respectively at 42m.


Dissolved O­2 – chlorophyll depth profile

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