Search

Zooplankton are an important component of marine systems and underpin many economically important fisheries. Open-ocean plankton communities are being affected by climate change but less is known about coastal systems. The Marine Strategy Framework Directive (MSFD) requires member nations to be able to detect changes in plankton community structure beyond those caused by natural variability. The paucity of knowledge on zooplankton dynamics in western Scotland hinders these legislative assessments. This thesis aims to fill this knowledge gap by a) providing a detailed description of mesozooplankton community dynamics in western Scottish waters, b) understanding the underlying drivers, and c) assessing the suitability of describing the plankton community of the western Scotland region by sampling at a single location. Data were primarily derived from Loch Ewe, a long-term plankton monitoring site within the Scottish Coastal Observatory that has been sampled every week since 2002. Loch Ewe displayed the typical seasonality of physiochemistry observed in temperate marine systems, although sea surface temperature (SST) and salinity declined over the time series. SST and chlorophyll 'a' explained the highest variation in zooplankton density, with their community composition being dominated by small calanoid copepods and appendicularians throughout the sampling period. Copepod diversity was significantly higher than in other UK waters, potentially reflecting higher chlorophyll 'a' concentrations and the advection of oceanic species into the site. Contrasting species specific responses to environmental variables within the same taxonomic groups highlighted the difficulty of pooling zooplankton taxa to represent aspects of ecosystem functioning. Comparison of the physiochemical properties and zooplankton communities at sites in western Scottish waters and in the Shetland Isles revealed that the latter was distinguished by higher salinities and lower SS By contrast, zooplankton communities sampled at disparate locations across the west coast could not be distinguished statistically, indicating that, at the taxonomic resolution and spatial scales examined, a single monitoring site is representative of the wider west coast region. This thesis provides two recommendations for future plankton monitoring: (1) The addition of a warm-water and cold-water copepod assemblage lifeform pair indicator within the UK Plankton Index is useful for discerning the effects of ocean warming. (2) The establishment of an additional zooplankton monitoring site in the Shetland Isles is required in order to meet MSFD assessment requirements.

In recent years, evidence has been found of Diel Vertical Migration (DVM) in zooplankton during the Polar Night in the Arctic Ocean. However, the drivers of this behaviour during an apparent lack of illumination and food are poorly understood, as is its spatial extent across the Arctic Ocean. A novel dataset comprising 58 deployments of moored Acoustic Doppler Current Profilers is used in this study to observe the vertical migratory behaviour of zooplankton on a pan-Arctic scale. Methods of circadian rhythm analysis are applied to detect synchronous activity. South of 75°N, DVM continues throughout winter (albeit with reduced vertical amplitude when compared to other times of the year). DVM is seen to cease for a short period of time (up to 50 days) at latitudes between 75° and 82°N. The duration for which DVM ceases is controlled primarily by latitude (and therefore the altitude of the sun), but is modified by sea-ice presence and other environmental parameters. A DVM pattern is not seen at 90°N at any time of the year, but ~365 day periodicity is detected in the surface backscatter levels at this latitude. During the Polar Night, the moon controls the vertical positioning of zooplankton across the Arctic. Aggregations at depth coincide with an avoidance of the surface for several (< 6) days over the full moon. The deepest aggregation is seen at 110 m, indicating a depth limit of moonlight perception. A new type of migratory behaviour is described: Lunar Vertical Migration (LVM) which exists as LVM-day (24.8 hour periodicity) and LVM-month (29.5 day periodicity) on a pan-Arctic scale. The strongest LVM-day is seen at 50 m depth. Sea-ice and cloud are found to modify both of these lunar responses. The results presented by this thesis show continuous activity throughout winter, and challenge assumptions of a quiescent Polar Night.

Two important factors controlling population abundance of mesozooplankton are reproductive success and predation. This research analyzed the physiological affects of bloom events, particularly the Texas brown tide alga Aureoumbra lagunensis and the red tide alga Gymnodinium breve on survival, feeding and reproduction of the copepod Acartia tonsa. From fieldwork, it appears bloom events could dominate other controlling factors such as temperature, salinity and chlorophyll abundance in determining A. tonsa fecundity. Laboratory experiments demonstrated that A. lagunensis and G. breve are not utilized effectively as food items, as indicated by low egg production values, however neither alga appeared directly toxic to A. tonsa. Behavioral and predation effects of natural and artificial increases in mesozooplankton pigmentation were measured. The egg bearing copepod Dioithona oculata was used to measure the effects of the presence of heavily pigmented egg sacs on predation. The resulting increase in image size led to a higher rate of predation on egg bearing D. oculata by the juvenile grunt (Haemulon aurolineatum). Neutral red was used to stain several species of mesozooplankton to see if the artificial increase in pigmentation would increase predation rates by visual predators. Of the four species examined only A. tonsa had a higher rate of predation of stained individuals