Station. 4

CTD downcast data for station 4 (figure 12) showed a two-tiered thermocline/halocline indicative of stratification. This was expected due to station 4 being the furthest offshore (figure 1) and therefore tidal influences are negligible.  The fluorometry data from the CTD (figure 12) and chlorophyll data (figure 13) showed a deep chlorophyll maximum (DCM) at 20m, corresponding to the second tier of the thermocline. ADCP data (figure 14) at this station revealed high levels of backscatter at depth of 20m, which corresponded with the chlorophyll maxima, suggesting a high zooplankton population grazing at this depth. Net samples supported this with both a large abundance and biodiversity of zooplankton recorded at this depth (figure 23). An oxygen minimum was observed at the same depth as the chlorophyll maximum (figure 13) due to the zooplankton population utilising oxygen through respiration. The low diversity of phytoplankton recorded can be attributed to predation within the area. The silicon and phosphate concentrations show a strong correlation throughout the water column (figure 13).


Figure 12.

Figure 13.

Figure 14.

Figure 15.

Station. 5

The CTD downcast for Station 5 displayed a well-defined thermocline and pycnocline showing strong stratification (figure 16). Fluorometric and chlorophyll maxima are evident on the thermocline at 17-19m (figure 17). Oxygen saturation showed a sharp decrease over the thermocline possibly due to zooplanktonic respiration with increased oxygen levels recorded below the thermocline. Silicon levels are low in the surface layers, and increase to a maximum value below the thermocline, indicative of significant silicon depletion in the surface waters. This can be attributed to the diatom population.


Figure 16.

Figure 17.

Figure 18.

Station. 6

Station 6 showed a partially mixed water column structure, with a less defined thermocline and pycnocline seen (figure 19). The water column became partially mixed due to the breakdown of the thermocline, caused by the increase in tidal effect as a result of being closer to shore. Phosphate, silicon and nitrate concentrations were all low at the surface, which correlated to high oxygen and chlorophyll concentrations (figure 20). As the chlorophyll concentration decreased with depth the oxygen showed a similar profile. This resulted in an increasing trend for all of the nutrients with depth, suggesting a lack of primary production. Low transmission throughout the whole water column suggested high turbidity (figure 19). The secchi disk measurements (figure 24) compliment the transmission data and reinforce the idea of high turbidity. The 1% light level calculated from the Secchi disk data was 24m.



Figure 19.

Figure 20.

Figure 21.

Phytoplankton:

Figure 22 shows the abundance of phytoplankton species at each of the 6 stations. The greatest abundance of phytoplankton was found at stations 1 and 6, located closest to the shore. The vast majority consisted of diatoms, of the genus, Chaetoceros, and to a lesser extent, various Rhizosolenia sp. The remaining 5% of the sample for stations 1 and 6 were made up of other diatom groups (Thalassiosira and Coscinodiscus) and dinoflagellates (Karenia mikimotoi, Dinophysis, Alexandrium, and Ceratium).

Station 3 had the third largest abundance of phytoplankton, at about 2000 cells per ml. This station was also third closest to shore. Diatoms were by far the most dominant genera, however there was a slight increase in the percentage of dinoflagellates. These sample sites were closest to the riverine nutrient inputs, explaining the observed higher phytoplankton abundance.

Further offshore, in stations 2, 4 and 5, phytoplankton counts are much lower. Diatoms remained dominant, although station 2 has a larger proportion of dinoflagellates, and rarer (at other stations) diatom genera, such as Thallasiosira. Stations four and five had higher proportions of large Rhizosolenia sp. suggesting these may be more adapted to scavenging the more limited nutrients further offshore. These low counts are at the same stations where zooplankton species are more abundant, and abundance may have diminished due to grazing.

Diatoms tend to outcompete other phytoplankton earlier in the bloom season, as they are better adapted to higher turbidity. All stations had fairly homogenous transmission profiles, which may favour diatomic plankton abundance down the water column. Whether these transmission levels are high, thus favouring diatom blooms, may be clarified by further study of time-series data.


Zooplankton:

Figure 23 shows zooplankton species abundance at each of the sampled stations. Station 4 had almost double the number of zooplankton members per m3 (11,000,000) than the next largest, station 5, the other far offshore station. Copepods are the most abundant zooplankton at all stations. Other abundant groups include Cladocera, Chaetognatha, Appendicularia, and polychaete larvae.

Stations 1-3 were closer to shore, and zooplankton abundances are much lower, at around 1,000,000 per cubic metre. Aside from copepods, hydromedusae and gastropod larvae made up a fairly large proportion of the zooplankton. Some of these less motile groups may be forced onshore by tidal forces, whereas copepods have greater control of their position, explaining their abundance at all stations. The lower abundance at the sites where phytoplankton is most abundant seemed unusual, as this would seem an ideal opportunity for grazing; however zooplanktonic population increases typically lag behind phytoplankton maxima.


Figure 22.

Figure 23.

Figure 24.

Conclusions:

From onshore to offshore the water column became more structured. This will be due to a decrease in tidal mixing processes which prevent thermal stratification occurring. The water column also became deeper, reducing the effect of wind on mixing. Progression further offshore saw the thermocline deepening due to this reduction in turbulent mixing as is typical of seasonally stratified seas. As Summer temperatures rise and thermal stratification increases, the onshore/offshore nutrient gradient is likely to show a marked increase.

The measurements of abundance and identification of phytoplankton and zooplankton samples were carried out by many different members of the group. This inconsistency may have given rise to some deviation in results on both counts. The stations all represent a snap shot in time, so before solid conclusions can be made a time series for each station would need to be established for further analysis.


Offshore Discussion, (Continued)

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