The well-known fate of CO₂ in aquatic ecosystems was widely documented in freshwater and marine environments. The pathways of CO₂ and its derivatives are fundamental information widely known in limnological, marine, wetland and river/runoff ecosystems (Figure 1). The significant long-term increase of atmospheric greenhouse gases, including CO₂, is undoubtedly accepted by the scientific community [1] . Carbon dioxide (CO₂) levels in the atmosphere rise

and more CO₂ gets absorbed into seawater and freshwaters as well [1] . The study of atmospheric CO₂ enhancement’s impact on the ecological trait of aquatic ecosystems was intensified recently (among others) [1] [2] [3] [4] .

Among inorganic carbon species, free CO₂ dominates in water at pH 5.0 and below, whilst above pH value of 9.5 Carbonate (CO₃) is quantitatively significant and Bicarbonate ( HCO 3 − ) is predominant at pH between 7.0 and 9.0. The Kinneret is indicated as hard water ecosystem with commonly > 8.0 pH caused by the release of the Hydroxyl ions as a result of CO₂ dissolving. Therefore, enhancement of dissolution, possibly caused by atmospheric “Greenhouse” gasses increase, might cause additional increase in pH.

Photosynthesis of living aquatic organisms (phytoplanktonic algae, submerged macrophytes, photosynthetic bacteria) and respiration of zooplankton, invertebrates, fishes, protozoa and plants affect the amount of CO₂ in aquatic ecosystems. Photosynthesis causing pH elevation and respiration effect is the opposite of pH decline. The final result, in a natural ecosystem, includes the involvement of all components.

During 1969-2001, the Lake Kinneret ecosystem has undergone ecological changes [5] . The prominent alteration was the change in phytoplankton community composition. The dominance of the bloom-forming Pyrrhophyte-Dinoflagellate Peridinium gatunenze was replaced by nano-plankton (Cyanophyta, Chlorophyta Diatoms). Previous studies [6] indicated a higher specific productivity rate of the small cells nano-planktonic algae organisms than that of large cells, net-palnktonic algae (Peridinium gatunenze). Zooplankton standing stock biomass was declining from 1969 to the mid-1990’s and increased afterwards.

The following processes were evaluated, aimed at the study of the impact of the increase of atmospheric CO₂:

1) Search for potential (headwaters and Hypolimnetic supply) sources for the enhancement of Carbon supply and Primary Production

2) Optional source for the Enhancement of CO₂ inputs caused by greenhouse gas increase.

3) The replacement of Peridinium gatunenze by nano-planktonic algae and consequently intensified Primary Production activity (Carbon Fixation).

4) DO, CO₂ and pH multi-annual and monthly fluctuations caused by photosynthesis enhancement.

All data of Primary Production (PP), Dissolved Oxygen (DO), pH and Alkalinity, Headwater nutrient inputs (monthly and annually) were taken from the Data Base and Annual (1970-2014) reports of the Kinneret Limnological Laboratory [7] .

Statistical analyses were taken from STATA 9.1, Statistics-Data Analysis: Simple linear predicted correlation and Fractional Polynomial (FP) predicted Regression.

Lake Kinneret is located in northern part of Israel, in the central part of the Jordan Rift Valley within the Syrian-African Graben. Its watershed (2730 km²) is situated between 32˚40' and 33˚38' North. Concentrations of DIC and DO in freshwater ecosystems depend on many factors in addition to ecosystem metabolism, among others, gas exchange with the atmosphere and inputs in precipitation, ground and surface water [8] . They [8] documented that diel cycles are controlled among others by exchange with the atmosphere. Lakes are sources rather than sinks of atmospheric CO₂ and are potentially important conduits for Carbon from terrestrial sources to the atmospheric sink as indicated by Cole et al. [9] . The increase of CO₂ dissolution into the liquid increased proportionally when CO₂ in the air rose, DIC availability was increased, photosynthesis was enhanced, and the photosynthetic energy cost decrease was documented by Qiu and Gao [10] . Yang and Gao [11] found significant enhancement of the growth rate of chlorophytes with CO₂ enrichment. Doubling of phytoplankton productivity as a result of doubling of the atmospheric CO₂ concentration was documented in [12] and in [13] . They [12] indicated that the doubling of atmospheric CO₂ concentration might result in an increase of productivity of more than 50% Freshwaters with low alkalinity and elevated atmospheric CO₂ may enhance aggravating phytoplankton bloom nuisance. Riebesell [14] documented confirmations of CO₂ supply limitation of Diatoms growth rate under optimal light and nutrient conditions. Yan et al. [15] concluded that enriched atmospheric CO₂ enhanced photosynthesis and growth of phytoplanktonic species which have a lower capacity of CO₂-concentrating mechanism. Recent studies showed that elevated CO₂ concentrations enhanced the growth of freshwater green algae [12] . The important role of inland waters as Carbon source to the atmosphere through evasion of greenhouse gases such as CO₂ and CH₄ and also as a sink through sedimentation was studied by [16] . Marks et al. [17] reviewed terrestrial Carbon export via inland aquatic systems as a key process in the global cycle, including out-gassing from lakes and fixation and sedimentation. Golub et al. [4] documented lake ponds and rivers that process large amounts of organic matter and some of which is emitted to the atmosphere as greenhouse gasses influencing climate change. Vaughan [18] reported [3] in The Guardian that in March 2015 CO₂ concentration in the atmosphere was 400.83 ppm and 406.82 and 404.42 ppm in December 2017 and December 2016, respectively. The long-term record monitored by NOAA indicates that between 1980-1998 and 1998-2016 CO₂ concentrations increased by 7.9% and 10.2%, respectively. A signal of precaution is given in [19] with regard to environmental changes including atmospheric deposition and climate, respectively, with consequences for eutrophication of freshwaters [13] . Prediction of increasing photosynthetic rates of herbaceous woodland species by elevation of atmospheric CO₂ is presented in [2] .

Several documented changes occur within the Kinneret ecosystem: changes of the phytoplankton assemblage composition of which the decline of Peridinium and enhancement of Cyanophyta, Chlorophyta and Diatoms are dominant; decline and reforming of the Zooplankton density; floods and dryness; extreme fluctuations of WL; Thermal fluctuations with respective changes of stratification span and timing; wide range of salinity changes; changes of the water utilization policy; onset and die-off of the beach vegetation; modified fishery management; and anthropogenic intervention of man-made changes in the drainage basin. None of these modifications was ever suggested to be correlated with the global case of increasing atmospheric CO₂ concentration. A tentative comprehensive evaluation is presented here aimed at correlating limnological fluctuations with increasing concentrations of atmospheric CO₂. As a test case the periodical term of 1970-2001 was chosen as having detailed records of relevant data. This period represents the lake conditions prior to major changes together with transition time towards the modified ecosystem and the beginning of the alternate situation. The key process is the enhancement of Carbon fixation by the photosynthetic production of organic matter after the early 1990’s (Figure 8). Consequently, sources of additional Carbon sources should be defined. Two options are most relevant: 1) through river inflows and/or 2) from the anoxic Hypolimnion during de-stratification following the existence of 8-month stable stratification. Quantitative evaluation of river inflows and hypolimnetic supply eliminate sufficient merit. Moreover, it is questionable how the high biomass of phytoplankton prior to the 1990’s maintained primary production. The answer is given by indicating the species biomass and its physiological features. The dominant algal species prior to the 1990’s was Peridinium. The Specific PP activity of this primary producer is lower than that of non-phyrrophytes which became dominant later. The need for enhanced Carbon supply following the species alteration became essential. On the other hand the external inputs declined (Table 1, Table 2) (Figures 1-4). Moreover, the external inputs routinely decline sharply in summer (Figure 5) when nutrients are mostly demanded, resulting in the creation of Kinneret summer steady-state ecosystem where food is limited for all food-web compartments. The most relevant option for additional supply of photosynthetic substrate is atmospheric CO₂ diffused input. An indirect indication for the CO₂ diffusion input enhancement is shown by the elevated pH (Figure 6) and consolidated by a similar increase of Alkalinity (Figure 7). What was the reason for the algal composition change? Peridinium requires plenty of Nitrogen which was mostly supplied from external sources in the drainage basin through river inputs. Due to anthropogenic implementations in the drainage basin, Nitrogen inputs declined dramatically and internal sources are absent, resulting in Peridinium reduction. The followers, the non-Phyrrophytes, especially nitrogen-fixers Cyanophytes, require much less Nitrogen, causing their predominance and the Phosphorus supply from external and internal sources not to decline. Figure 8 represents these developments: the Nitrogen elimination caused the decline of Peridinium and its Primary Produced products, whilst later non-phyrrhphytes predominated and Primary Production was elevated (Figure 8), causing the increase of DO (photosynthetic product) concentration (Figure 9). It is likely that, due to the enhanced Photosynthetic activity, CO₂ input flux was enhanced, but its concentration in the water declined as a result of algal intake. The positive interrelation between intensified PP and DO elevation is shown in Figure 11. The influence of PP enhancement on CO₂ uptake and its remains is shown in Figure 12: The higher the PP is the lower the CO₂ concentration in the water. These combined effects are consolidated in Figure 13 and Figure 14: pH is elevated proportionally to PP activity (Figure 13) and CO₂ concentration decline (Figure 14). The exclusive dependence of Primary Production by the Kinneret Phytoplankton on atmospheric pCO₂ is presented in Table 4.

Conclusively, with respect to global trends of atmospheric gas changes [19] , it is suggested that as a result of a combination of modified conditions and increase of atmospheric “Greenhouse” gasses, CO₂, the eutrophication threat in the Kinneret ecosystem was probably aggravated. If predictions are correct and Greenhouse atmospheric gasses increase, the management of Lake Kinneret and its drainage basin should be thoroughly reconsidered.

Major changes of several parameters were altered within the Lake Kinneret ecosystem. External Nutrient fluxes were lowered significantly probably including dissolved inorganic Carbon. The epilimnetic pH and DO levels increased and consequently the rate of Primary Production increased. Independently, atmospheric “Greenhouse” gasses (CO₂, NH₄) increased. The photosynthetic activity of the planktonic algae was intensified since the early 1990’s. Carbon demands for PP enhancement followed by pH and DO increase might be supplied from diffusion of atmospheric sources. If Greenhouse gasses continue to be elevated in future, the Eutrophication in Lake Kinneret might be realistic and require reconsideration.

Gophen, M. (2018) Atmospheric CO₂ Increase: A Potential for Primary Production Enhancement in Lake Kinneret. Open Journal of Modern Hydrology, 8, 50-65. https://doi.org/10.4236/ojmh.2018.82005