The relationship between water quality and aquatic organisms in tidal lakes, Medan-Indonesia

Siombak Lake is a tidal lake in Indonesia. The dynamics of the lake’s waters experience two high tides and two low tides within 24 hours (except during neap water). Groups of freshwater, brackish, and marine organisms make up Siombak Lake’s aquatic biota, which varies with the seasons and tidal conditions. The purpose of this study is to clarify how aquatic biodiversity in lakes is impacted by tidal dynamics. This research was conducted at Lake Siombak, Medan City, North Sumatra Province. Data collection was carried out from September 2018 to August 2019. The analysis carried out was a multivariate PCA analysis assisted by Microsoft Excel 2016 and Minitab. The research results show that tidal dynamics cause fluctuations and dynamics in lake water quality and lake biological communities. Plankton are aquatic organisms greatly influenced by water dynamics and water quality. Phytoplankton abundance is influenced by sunlight and nutrient levels, while TSS, salinity and phytoplankton abundance influence zooplankton abundance. The abundance and presence of fish are closely related to their food sources, namely zooplankton, crustaceans, and mollusks. The existence of these tidal dynamics provides sustainability and stability to the environment and ecosystem of Siombak Lake


Introduction
Siombak Lake is a tropical coastal lake in Medan, North Sumatra Province, Indonesia.This lake is a tidal lake, with the dynamics of its waters governed by the Malacca Strait tides [1,2].Siombak Lake covers 29.72 hectares and has an average lake depth of 1.63 -4.53 m during rainy days (mean tide = 3.27 m) and 1.61 -4.38 m during dry days (mean tide = 3.25 m).Except during dead tide (neap tide), variations in the water level of Siombak Lake occur every 6 hours (experiences two high water and two low water in a day).Because of the tidal cycle, the Lake Siombak's water level changes throughout the year.In November 2018, the greatest water level in Siombak Lake was 227 cm.The lowest water level was recorded in February 2019 at -24 cm [2].
Siombak Lake is quite unique and different.The lake water system is an estuary.The river flow that enters the lake seven kilometers from the Belawan River Estuary (Malacca Strait) is still influenced by ocean tides [2].Some mangroves develop around the lake's perimeter [3,4].The aquatic biota of Siombak Lake consists of groups of freshwater fish, brackish, and marine creatures [5][6][7][8].Siombak Lake serves as catchment area for water, a flood control zone, a fishing area for fish, shellfish, and crabs, and a tourist attraction.There are additional pond fisheries, residential areas, industries, and the Terjun Medan City Landfill in the area around the lake [9].
Various studies of Siombak Lake have been conducted, including on morphology and hydrodynamics as well as the status and profile of lake water quality [1,2,10,11].Studies on aquatic biodiversity such as fish, crustaceans, mollusks, and plankton have also been conducted [3][4][5][6][7][8].In addition, studies on mangroves and bird and mammal biodiversity have been reported in 2020 and 2021 [3,12].However, studies and discussions related to the relationship between water quality dynamics and lake aquatic biodiversity need to be reported.The purpose of this research is to explain the impact of tidal dynamics on the lake's aquatic biodiversity

Study site
This research was carried out at Siombak Lake in Medan City, North Sumatra Province (Fig 1).Data was collected between September 2018 and August 2019.

Data collection
Data collection on water quality and aquatic biology is carried out every full moon at high and low tide.Water quality data refers to water biology and is further processed from previous research by [2,4,5,6,7,8,10].

Data analysis
The analysis conducted was PCA multivariate analysis assisted by Microsoft Excel 2016 and Minitab.

Aquatic biodiversity
The Siombak Lake ecosystem has a fairly high and complete biodiversity.Biodiversity in Siombak Lake consists of 54 genera of phytoplankton, 12 genera of zooplankton, 9 families of nekton larva and juvenile, 10 types of mollusks, 18 types of crustacean, 27 types of fish, 14 types of mangroves, 12 types of birds, dan 2 types of primates (Table 1).The lake has a biological feature that is rarely encountered elsewhere: diverse mangrove trees, dominated by Nipa (N.frutican) and Berombang (S. caesolaris), grow on its numerous sides (borders).Other notable species in Siombak Lake include herring fish (M.cyprinoides), eel fish (A.bicolor), and jellyfish (A.aurita).Siombak Lake's biological community includes fresh, brackish, and marine biota, as well as newcomers and settlers [3,4,5,67,8,12].

Linkage of habitat and water quality to biodiversity
The level of water in Siombak Lake is directly connected to various factors such as current, salinity, total dissolved solids (TDS), organic matter, and nutrients.When water flows into the lake, it transports organic matter and salinity from the sea, resulting in a higher concentration of these elements in Siombak Lake.TDS and salinity have a strong correlation with each other, reaching 0.97 during high tide and 0.85 during low tide.High TDS levels in the water lead to an increase in salinity concentration, making TDS and salinity closely related to quality parameters.Salinity, temperature, pH, and DO are water quality characteristics in Siombak Lake that have a relationship with aquatic biology.At high tide, salinity correlates with phytoplankton, the number of larval families, the distribution of mollusk species, the abundance of crustaceans the distribution of crustacean species, and the distribution of fish species.The abundance of aquatic life in Siombak Lake is linked to its salinity levels.When the salinity is low, freshwater species are dominant, while brackish/marine organisms dominate in high salinity conditions.The temperature in the waters of Siombak Lake is closely correlated with zooplankton, larvae, mollusks, and fish.DO and pH showed a close correlation with mollusks and fish.
The results of spatial PCA analysis showed that in both high tide and low tide conditions aquatic organisms (except plankton and larvae) were influenced by the basic characteristics of water quality, namely water temperature, DO, and pH (Figure 2-3).The presence of these organisms also characterized stations 3-6 at high water and stations 3 and 5 at low water.This indicates that at high water the organisms in Siombak Lake are more abundant in the northern and eastern parts, while at low tide more are found in the shallow areas at stations 3 and 5.Meanwhile, plankton itself both at high water and low water is influenced by nutrients presence, brightness, turbidity, and organic matter.The presence of plankton characterizes Station 1 in both high tide and low tide conditions.Furthermore, larvae and juveniles are somewhat different among high water and low water, which at high tide are influenced by pH, DO, and temperature and characterize stations 3-6.At low tide, larvae and juveniles characterise station 1 which is influenced by the presence of plankton.

Linkage of tidal dynamics to aquatic biota communities
The results of temporal PCA analysis (Figures 4-5) show that the presence of aquatic organisms is more dynamic than spatially, following the highly dynamic conditions and characteristics of water quality based on time conditions.Molucca is influenced by turbidity and characterizes the transitional months of May and June.Feb-March is a time when aquatic organisms are very dominant and characterize both months, the presence of fish, crustaceans, larvae, and juveniles as well as plankton.This is not surprising as fish are especially abundant during this month, characterized by the presence of halfbeak fish, snapper, and spotted scat fish as well as the abundance of Ambassies fish.Meanwhile, larvae were also abundant, especially the post-larval phase of jellyfish, which was particularly abundant in March.Spatially, phytoplankton and zooplankton were highly correlated with moderate (0.53) to strong (0.85) correlations.Temporally, the relationship was lower than spatially (r<5).However, when plotted in PC1 and PC2 phytoplankton and zooplankton remain on the same axis and close together, as seen in Figure 4-5.Similarly, fish are spatially and temporally closely related to their food sources, namely, zooplankton, crustaceans, and mollusks.The waters of Siombak Lake are a food search area for fish organisms, both fresh and marine fish that alternately enter the lake according to salinity conditions [5,8].Based on correlation and regression and the descriptive analysis between salinity and aquatic organisms in Siombak Lake, it can be said that salinity influences the distribution and number of plankton., larvae, crustaceans, and fish species and populations.While mollusks themselves are more influenced by the characteristics of DO, pH, TSS, and turbidity values in the waters.
Salinity, temperature, pH, and DO are water quality characteristics in Siombak Lake that have a relationship with aquatic biology.At high tide, salinity correlates with phytoplankton, the number of larval families, the distribution of mollusk species, the abundance of crustaceans the distribution of crustacean species, and the distribution of fish species.The presence of aquatic biota species in Lake Siombak is closely in relation to the level of salinity which in low salinity conditions will be dominated by fresh biota, and vice versa in high salinity will be dominated by brackish/marine organisms.The temperature in the waters of Siombak Lake is closely correlated with zooplankton, larvae, mollusks, and fish.DO and pH showed a close correlation with mollusks and fish.Phytoplankton in Siombak Lake correlated with salinity, turbidity, brightness, currents, zooplankton, and larvae.The intensity of sunlight entering the water is one of the important factors in the phytoplankton community [8,13,14].Sunlight is needed in the process of photosynthesis in water [15,16].
The population and "presence" of fish in Siombak Lake are influenced by water quality (DO, pH, temperature, and salinity) and food sources (plankton, mollusks, and crustaceans).DO, pH, and temperature are important parameters in the life of aquatic biota in aquatic ecosystems, including coastal lakes [17].Another parameter that affects the fish population in coastal lakes is salinity.Salinity dynamics in coastal lake waters play a role in fish species' different 'presence'.High salinity conditions will be dominated by the presence of marine fish, and vice versa in low salinity will be dominated by freshwater fish [17,18,19].
Another factor affecting the prevalence and presence of fish species in Siombak Lake is the availability of food in the lake, particularly plankton, mollusks, and crustaceans.Spotted Barb, Tilapia, and Mozambique Tilapia fishes are common plankton-eating fish in Siombak Lake.Indo-Pacific tarpon fishes are also fish that are often found in Siombak Lake.Indo-Pacific tarpon fish are plankton feeders (especially zooplankton) and eat crustaceans.Other fish that feed on crustaceans are Northern mud gudgeon and Scalloped perchlet fish, which are commonly found in the dry season [20,21,22].

Conclusions
The occurrence of tidal dynamics causes fluctuations and dynamics of lake water quality and the dynamics of the lake's biological community.Plankton is a type of water organism heavily influenced by dynamics of water and quality of water.Phytoplankton abundance is affected by the presence of sunlight, nutrients and amout of zooplankton.Zooplankton abundance is influenced by TSS, salinity and phytoplankton abundance.The abundance and presence of fish are closely related to their food sources, namely, zooplankton, crustaceans, and molluscs.

Figure 2 .
Figure 2. Principal component analysis of water quality characteristics and aquatic organisms spatially in Siombak Lake at high tide.

Figure 3 .
Figure 3. Principal component analysis of water quality characteristics and aquatic organisms spatially in Siombak Lake at low tide.

Figure 4 .
Figure 4. Principal component analysis of temporal characteristics of water quality and aquatic organisms in Siombak Lake at high tide.

Figure 5 .
Figure 5. Principal component analysis of water quality characteristics and aquatic organisms spatially in Siombak Lake at low tide.

Table 1 .
Aquatic and terrestrial biodiversity in tidal lake.