Exploring phytoplankton dynamics in Whangateau Harbour, New Zealand and the influence of benthic grazing, water parameters, and tidal cycles
Exploring phytoplankton dynamics in Whangateau Harbour, New Zealand and the influence of benthic grazing, water parameters, and tidal cycles
Abstract
This study explores the variations in plankton characteristics across depth gradients, salinity, and temperature ranges during different tidal phases in Whangateau Harbour, New Zealand. Phytoplankton, as primary producers, play a crucial role in marine ecosystems, influencing food web dynamics. Samples collected on January 30th, 2024, revealed distinct differences in plankton distribution and chlorophyll concentrations between incoming and outgoing tides. Outgoing tide plankton, found at deeper depths with lower chlorophyll levels, displayed a broader salinity range and was abundant at higher temperatures than incoming tide plankton. Our results suggest potential influences from nutrient shifts associated with tidal activities and temperature variations, impacting plankton depth distribution. Higher chlorophyll concentrations in incoming tide plankton may be linked to sun exposure and reduced planktivory. However, negative chlorophyll values in the dataset require caution due to potential instrument and human errors. The findings support the hypothesis that decreased chlorophyll concentrations result from benthic grazing following plankton and nutrient transport from the incoming tide.
1. Introduction
As primary producers, phytoplankton form the foundation of marine food webs, supplying vital resources for benthic grazers. Understanding the dynamics of plankton abundance, distribution, and chlorophyll concentrations is essential for comprehending ecosystem health and productivity, particularly in coastal environments like Whangateau Harbour. Samples were collected from Whangateau Harbour, New Zealand, on January 30th, 2024, using the University of Auckland's research vessel. Two groups of UOA Marine 305 students conducted the sampling, one allocated to the incoming tide (around 8:00 am) and the other to the outgoing tide (around 2:30 pm). We hypothesise that decreased chlorophyll concentrations result from benthic grazing following plankton and nutrient transport from the incoming tide. 2. Methods and Materials
2.1. Sample equipment and collection
Plankton samples were collected using a plankton net with a mesh size of 20 μm, which was deployed and retrieved manually by students. A hand-held conductivity temperature depth (CTD) sensor was deployed simultaneously to measure the temperature, salinity, and depth at which the plankton was collected. A fluorometer was used to assess the abundance of phytoplankton at the study sites, and a spectrophotometer was used to measure chlorophyll concentrations. 2.2. Microscopic examination and identification
The collected samples were placed onto microscope slides and examined. Organisms were identified and recorded, including centric diatoms, diatom chains, pennate diatoms, and dinoflagellates. Additionally, the sample was observed for copepods, larvae, gelatinous zooplankton, and debris relevant to the harbour environment (depth, salinity and temperature) from which it was collected.3. Results
Plankton distribution across depth gradients indicated that specimens from the outgoing tide were found at deeper depths, averaging -1.869m from the surface (Fig. 1a - 1c). In contrast, plankton from the incoming tide inhabited shallower depths, averaging -1.267m. Plankton samples collected from the outgoing tide exhibited lower chlorophyll concentrations compared to those from the incoming tide, measuring at an average of 0.239 and 0.024 (Fig. 1a). Notably, plankton from the outgoing tide displayed multiple negative values of chlorophyll (>200), while the incoming tide samples showed significantly fewer negative values (<10). Additionally, plankton from the outgoing tide demonstrated a broader distribution across salinity gradients, covering an extensive range (35.102 - 35.285) and averaging 35.174 (Fig. 1b). In contrast, plankton from the incoming tide exhibited a narrower salinity range (35.141 - 35.231) but had a higher average salinity at 35.207.Further observations revealed that plankton from the outgoing tide was more abundant at higher temperatures, with an average of 24.50°C and a broader temperature range of 24.40°C - 24.70°C (Fig. 1c). Conversely, plankton from the incoming tide was observed at lower temperatures, averaging 22.17°C, with a narrower temperature range of 22.16°C - 22.19°C.Fig 1. The water parameters measured at depth (m) during incoming and outgoing tides were recorded at Leigh in January 2024. A = Chlorophyll (mg/3), B = Salinity (measured using conductivity), C = Temperature (C°). 
Fig 1. The water parameters measured at depth (m) during incoming and outgoing tides were recorded at Leigh in January 2024. A = Chlorophyll (mg/3), B = Salinity (measured using conductivity), C = Temperature (C°).4. Discussion
4.1 Plankton dynamics and effect on benthic grazers
With plankton abundance occurring deeper during the outgoing and shallower during the incoming tide, there may be potential influences from nutrient shifts associated with upwelling, downwelling, and water mixing in the water column (Villamaña et al., 2017). Considering that the incoming tide occurred around 8:00 am and the outgoing tide around 2:30 pm, it is plausible that temperature variations contribute to these depth differences. The reduced sun exposure during the incoming tide likely led to lower temperatures, while the outgoing tide experienced prolonged sun exposure, resulting in higher water temperatures. This temperature gradient could further impact plankton depth distribution, as sunlight is crucial for energy production (Kamykowski & Zentara, 1977).Higher chlorophyll concentrations in plankton from the incoming tide can be linked to elevated salinities and lower levels of planktivory (Barnes & Wurtsbaugh, 2015). Suspension-feeding bivalves in Whangateau, such as pipis (Paphies australis) and cockles (Austrovenus stutchburyi), have been observed to exert diminishing effects on phytoplankton abundance due to extensive filtering (Jones et al., 2017). This could explain the lower chlorophyll levels during the outgoing tide, as benthic grazing via planktivory can decrease chlorophyll concentrations (Carpenter & Pace, 2018).4.2. Potential discrepancies
Regarding the negative chlorophyll values in the dataset, the possibility of both instrument and human errors contributing to this issue should be acknowledged. Since the data collection and analysis were conducted by students, potential inaccuracies due to their relative lack of experience need to be considered. A remeasurement on February 1st, 2024, was undertaken due to broken instruments during previous sampling. While negative chlorophyll values are theoretically impossible, they remain in the dataset to acknowledge the potential for errors and prevent bias resulting from assessing only positive values.4.3. Conclusions
This investigation into plankton interactions with benthic grazers at Whangateau has enhanced our understanding of the importance of tidal influence. Our findings support the hypothesis that decreased chlorophyll concentrations result from benthic grazing following plankton and nutrient transport from the incoming tide.References
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