Falmouth Group 8

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The plot of chlorophyll concentration against depth shows that there are more chlorophyll concentrations above 1µg/L in the first 10m than in deeper waters. The plot shows an average decrease in chlorophyll concentration with depth. There are three chlorophyll concentration points on the graph at 4.4m, 8.9m and 9.4m with concentrations of 9.2µg/L, 7.2µg/L and 9.4µmol/L respectively, these appear to be anomalies when compared to the rest of the plots.

This distribution of chlorophyll with depth agrees with the general trend of higher phytoplankton concentrations in the surface waters where the light can penetrate.

The plot of chlorophyll concentration against salinity shows three anomalies, which lie at salinities of 31.1, 32.2 and 34.8 with chlorophyll concentrations of 9.4µg/L, 9.2µg/L and 7.2µg/L respectively. Ignoring these anomalies the other plots show a decrease in chlorophyll concentrations with increasing salinities, decreasing from about 3.5µg/L at salinity 29.1 to 1.13µg/L at salinity 35.


Chlorophyll-Figures 1&2

The figure 5 displaying the dominance of phytoplankton reveals that diatoms comprise 50% of the sample from station 1, increasing up to 80% from station 2’s sample and completely dominating the sample from station 3. The replicate for station 3 however contained dinoflagellates although they only comprise <5% of the sample. Station 4 only contained dinoflagellates and diatoms, the latter of which were still dominant. Only diatoms were detected in the sample from station 5 and station 8 contained diatoms and dinoflagellates in similar proportions to station 4.

There was no single zooplankton group dominating the sample from bottle 1, with only copepoda dominating at a proportion of over 30%. There were similar proportions of copepod nauplii, hydromedusae, gastropod larvae and ctenophora (up to over 15% each). In bottle 2 copepod nauplii occur in a significant majority at approximately 60%. Whilst they still comprise the majority of the sample in bottle 3, it is at a reduced proportion of approximately 30%. At a combined total of 40% cirripede larvae, hydromedusae and gastropod larvae comprised the sample.

Both the abundance and diversity of phytoplankton exhibit a positive trend with increasing salinity. A reduction in each can only be observed to occur between salinites 29-33 and 34.5-35. The salinity range of 29-33 however has relatively fewer data points to the salinity range 33-35. The overall range in phytoplankton abundance is from around 200-1200 cells per litre. The range for phytoplankton diversity is from around 4-14 species per litre. Interestingly from salinity 33 the abundance and diversity of plankton follow a very similar pattern to each other, making it appear to be that each had a positive relation to the other variable.

The figure for the zooplankton reveals a different pattern, with diversity now following a negative trend in relation to abundance and vice versa. From salinity 29-34 zooplankton abundance increases up to approximately 35,000 animals per litre, but this is then reduced to around 7,500 animals per litre at salinity 35. Meanwhile from around 12 species per litre at salinity 29, zooplankton diversity decreases down to 9 species per litre at salinity 34. When abundance decreases at salinity 35, diversity likewise increases up to 13 species per litre. It must be noted however that relatively few samples are available for comparison for zooplankton.

Diatoms consistently are the dominant phytoplankton group throughout the estuary, and the greatest diversity in phytoplankton groups occurred further down the estuary. Given that phytoplankton are predominantly marine species this seems to be a reasonable scenario. Different plankton groups may exhibit different tolerances in their salinity range, with the euryhaline groups being found further up the estuary. The saline end of the estuary in turn provides salinity conditions favourable to most groups of phytoplankton.On the other hand only one of the zooplankton bottles displayed significant dominance from a particular zooplankton group, and generally no single group dominates the sample as much as with the phytoplankton. No trend was apparent along the length of the estuary, although the few samples collected give a limited scope for analysis. Zooplankton movements are more sporadic and not governed by abiotic factors as strongly as with phytoplankton. They will often gather where there is an abundance of phytoplankton so a reduction in diversity could indicate the more limited food supply available to them in different areas of the estuary. It is also notable that larvae are quite prevalent in the more diverse sample bottles, which may be linked to the local mussel farms and the fact that different areas in an estuary are used as a nursery by various marine biota.

The patterns in zooplankton and phytoplankton populations throughout the estuary revealed some distinct contrasts. The greater the abundance of phytoplankton species at the time the lower the diversity was, whereas with zooplankton a greater abundance generally revealed high levels of diversity. This could be explained by the differences in adaptations of the two organisms. Phytoplankton, being primary producers, are mostly adapted to abiotic conditions such as light levels and nutrients. This leads to distinct blooms being formed during various seasonal periods, when even a single order of phytoplankton may dominate, being most adapted to the conditions found at that time. Zooplankton on the other hand are consumers and so once again the available food supply may be reflecting the levels of diversity resulting in the estuary.


Plankton Figures 3-6

Plankton in the Fal Estuary

Click graphs to enlarge

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