Title: Exploratory analysis of seasonal variation of total
phosphorus and lake water temperature in Bosherston Lakes.
No other element
in fresh waters has been studied as intensively as phosphorus (Wetzel,
2001). A very large proportion of
phosphorus, greater than 90% occurs as organic phosphates and cellular
constituents in the biota. (Wetzel, 2001).
phosphorus content of water consists of phosphorus in the particulate and in
dissolved components (Juday,1927; Ohle,1938; Wetzel, 2001).
The Bosherston lakes are artificially
created lake systems located in the Stackpole estate, south-west Wales,
National grid reference 9794. The lakes were formed by progressive flooding of
the three limestone valleys eventually comprising a central body of water
having eastern, central and western arm. The lakes form a part of the Stackpole
Nature Reserve and are owned by the National Trust. The lake system is designated
as a special area of conservation (SAC) under the Habitats Directive as the
Annex I habitat “Hard oligo-mesotrophic waters with benthic vegetation of Chara spp” (CCW report no.858). The lake
water quality depends on the water quality constituents within groundwater and
surface water inflows, internal release from, and deposition of constituents
into lake sediments and the biological uptake of the lakes.
Most of the phosphorus, which arrives in
the lakes, is attached to sediments. A lot of sediment arrives from the
catchment due to the arable and poached grassland. Small amount of sediment is trapped
by the sediment ponds, upstream of the One arch bridge and partly by the
reedbeds (CCW no.858). Majority of it enters the Upper east arm due to the
stormflow. Although, uncertain amounts enter via groundwater from slurry
applications to land. Phosphorus also enters due to internal P cycling within
the internal sediments of the lake and the pond (CCW Contract science report
no.858). This report aims to assess the seasonal variation of 10 year-long
datasets from 2000-2009 of total phosphorus levels in the Stackpole stream
(input stream), Upper east arm, Central arm, Western arm and Central lake and
the effect of climate change by correlating the water temperature with total
phosphorus levels in the lake system.
The dataset available for total phosphorus levels was
sampled across five main sites around the lakes. The Stackpole stream, the
Upper East arm, Central arm, Western arm and the Central lake (Figure2.1).
availability of continuous majority of data and analysing the water quality of
recent past years was the prime reason for choosing the dataset from 2000-2009.
Data representing a value <20 has been taken as 25 for all the analytical tests. Mean values of sets of three respective months are determined by measuring mean seasonal value. 2.2 A peak is observed (Figure 2.2) in the spring of year 2000 where mean total phosphorus levels reached a maximum of 157 ug/l and the highest mean water temperature reached 16.3 in the summer of 2003. Phosphorus levels in the Central lake mainly show low levels, although a mean value of 292ug/l was observed in autumn 2009. Upper east arm showed highest levels of phosphorus compared to all other sites. The highest value observed equals to 324.66ug/l in summer of 2006. 2.3 2.4 Statistical analysis was performed to figure out the relationship between total phosphorus levels and temperature. 3. Results: 3.1 Comparing long term and seasonal variations of total phosphorus in the Bosherston lily ponds: The seasonal average total phosphorus concentrations exhibit large variations. On an average, higher values of phosphorus were observed in the spring and summer months. The highest value of seasonal concentration of phosphorus was observed in the Upper East arm in summer 2006. The Central lake recorded the lowest value of total phosphorus in spring 2005. Majority of the fall in phosphorus concentrations in observed in winter months. Surface sediments are more oxidised during the winter months which act as a barrier to phosphorus release due to precipitation of phosphorus by ferric ions (Wilkinson 1985showed significant drop in temperatures in all the). Considering the water temperature, the winters of 2003 five sites, where, the Central lake recorded minimum average temperature of 3.4°C. The total phosphorus concentration levels correlate significantly with increasing water temperature, whereas some values are independent of temperature. The Pearson and Spearman rank correlation coefficients of the Upper east arm and the Central arm show positive relationship between the increasing total phosphorus concentrations and increasing temperature, although no significant trend is observed for the other sites. Studies suggest that the biomass of phytoplankton increases with the increase in water temperature and phosphorus (Guo et al., 2017). The phosphorus release experiments using Bosherston sediment (Niciecki, 2008) show that increased temperatures can stimulate release of phosphorus from sediments (Spears et al 2007, Kim et al 2003 & 2004; CCW no.858). Increase in lake temperature can have more effect on the Eastern arm as the water has more iron content (CCW no.858). Lake water temperature has also explained 70% seasonal variation in the study of four shallow lakes in Denmark (Jensen and Andersen 1992). A lot of absent monthly data has resulted skewing in the seasonal representation of concentrations, as in some cases a single month was representative of the entire season. Discontinuity resulted in the graphs due to unavailability of data, therefore, there is lack of significant representation of water temperature and phosphorus concentrations for some seasons. 4. Discussion: 4.1 The Eastern arm is fed mainly by three streams including the Stackpole stream. The one arch swamp acts as a sediment trap for this stream explaining the higher levels of phosphorus concentrations (Poole and Haycock, 1993). Phosphorus is a key performance indicator for the conservation of lake. In the Eastern arm the mean annual Phosphorus levels should be <50ug/l (CCW report no.858). The sediments in the Eastern arm and Central lake are saturated with phosphorus because little is absorbed by the sediment and phosphorus builds up in the pore water and readily mixed with the water column above(Rees,1989). 4.2 Total Phosphorus and eutrophication: Climate change is expected to increase eutrophication risk in freshwaters and total phosphorus level is considered to be the primary driver of eutrophication (Charlton et al., 2017). The Central lakes and the eastern arms experienced intense algal blooms in 1976. Pipeline diversion was carried out to decrease the nutrients (treated sewage) in 1984 (Paul Culyer,NRW). These incidences denote the fragility of the lake waters to total phosphorus concentrations. Cyanobacterial blooms are expected to become more frequent in freshwaters due to eutrophication and climate change effects (Charlton et al., 2017). The increased level of phosphorus in the summer months due to nutrient release causes harmful effect on the Chara species and increases the rate of algal blooms (NCC, 1988). Warmer temperature can lead to increase in phosphorus release from the lake sediment stores, exposing the lakes to eutrophication (CCW report no.858) 4.3 Environmental management in the lakes: The CCW report 858 mentions short term and long-term management strategies for the Bosherston lakes. Silt dredging or pumping is an adopted method to remove the phosphorus rich upper sediment layers and should be considered in the Eastern arm to restore the clear water conditions (CCW report no.858). Activities like working with the Environment Agency and the local community can help reduce inputs of phosphorus from point sources such as septic tanks, manure heaps and the non-point sources like liquid slurry application in agricultural fields (CCW report no.858). 4.4 Bosherston lakes water quality: 4.1 The Bosherston lakes are mainly utilised for fishing and other recreational activities. The lake is not utilised for drinking water purposes. Although water quality has to be maintained for the existing flora and fauna of this freshwater charophyte lake. The total phosphorus targets for freshwater lakes by the Water Frame Directive (WFD) state that the annual mean total phosphorus levels for oligotrophic lakes of all kinds of depth should be 10ug/l (JNCC, 2015). According to the Environmental Agency's GQA classification, the Eastern arm falls into the category of very high total phosphorus level as the mean annual level in Eastern arm >50ug/l.
of long-term datasets of total phosphorus and the water temperature of the
Bosherston lakes displayed that variations showed clear seasonal trends in the
concentrations. The correlation of water temperature with the total phosphorus
concentrations exhibited majority increase in the phosphorus levels as the lake
temperature increases resulting in higher concentrations during summer. The
Eastern arm is affected the most compared to the other five sites. The results
also demonstrated that future climate change effects would cause major changes
in the water quality of the lakes due to algal bloom. Hence, environmental
bodies should continue their efforts to remove total phosphorus pollution from
the lakes to maintain the necessary ecological standards.
Becks Pond Action Plan DELAWARE DIVISION
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