Even though aquaponics is the production system that is increasingly gaining focus, there is an absence of publications studying the use of pesticides in aquaponics. Therefore, the aim of this study was to assess the negative effects of one synthetic (chlorpyrifos) and two botanical insecticides (azadirachtin and pyrethrin) to matured biofilter in plastic buckets.
The effectiveness of biofilter bacteria is determined indirectly, as concentrations of ammonia, nitrites, and nitrates in the water were measured at 0h, 5h, 13h, 21h, 29h, 37h, 45h, and 53h after insecticides application. The results showed negative effects of azadirachtin on the first step of nitrification, as the concentration of ammonia was higher compared to other groups for almost every sample point.
Negative effects on the second step of nitrification and higher concentrations of nitrites in the water were also detected, but no statistical differences were observed in the present study, due to the large variation between buckets. Nitrates were lower in water treated with azadirachtin from 29 hours from the start to the end of the experiment. Application of the other two insecticides to biofilter did not cause any effects and showed no difference compared to the control group.
Aquaculture is the fastest-growing food production sector during the last 40 years (Troell et al., 2014). With an average annual rate of 7.8% in the period 1990-2010, it exceeded all other sectors of livestock production (Troell et al., 2014).
Today, fish produced in aquaculture account for more than half of the total consumed fish worldwide (Blanchard et al., 2017). To achieve sustainability of aquaculture production during this period of remarkable growth, integrated culture systems gained focus, and one of the most popular integrated systems is an aquaponics (Turcios & Papenbrock, 2014).
Aquaponics represents a combination of recirculating an aquaculture system (RAS) and cultivating plants in the water (hydroponics). The general idea of integrating these two systems began during the 1970s (Junge et al., 2017), and since then, aquaponics production is steadily growing and used not only in family farms but also at commercial scales.
It serves as the primary source of income for around 30% of farmers who own aquaponics in the United States (Love et al., 2015), and is also getting more popular in Europe (Villarroel et al., 2016). There is now an emerging possibility of transforming food production in developing countries, especially ones having subtropic or tropic climates (König et al., 2016).
Aquaponics is often advertized as eco-friendly production and there are several recommendations for non-chemical methods and the use of integrated pest management in aquaponics (Goddek et al., 2015; Sirakov et al., 2016).
On the other hand, after long-term use of the same aquaponics system, the presence of pests and diseases is increasing and the use of pesticides could be considered (Bittsánszky et al., 2015). Pesticides are very effective against insects, bacterial, fungal, and viral diseases that could affect plants, but due to their chemical structure, they can be toxic to fish and other aquatic organisms. Some groups of pesticides, like pyrethroids, are extremely toxic to fish, with mean lethal concentrations (LC50) below 10 μg L-1 (Bradbury & Coats, 1989).
Another issue is that synthetic pesticides are more persistent comparing to botanical and can remain in aquaponics for a long time. This represents a significant problem in the consumption of fish from aquaponics, since most of the synthetic insecticides are lipophylic, easily soluble in biological membranes, penetrating fish organisms, and later stored in lipid or muscle tissue.
Available alternatives to synthetic insecticides are botanical insecticides, extracted from plants, acting as neurotoxins toward insects, with the main feature being that they are easily biodegradable in water and when exposed to sunlight. Examples of these types of insecticides are: azadirachtin, extracted from neem (Azadirachta indica) seeds, and several types of pyrethrin compounds, extracted from chrysanthemum (Chrysanthemum cinerariifolium) plant. These insecticides are widely and routinely used as plant production products in traditional crop science, as well as in hydroponics (Ujváry, 2010), and this was the rationale for their use in the present study.
Pyrethrum and azadirachtin are believed to be the most widely used botanical insecticides globally, but accurate usage per state or continent cannot be found in the FAO statistics or in scientific publications (Isman, 2020). It also has to be stressed that pyrethrum is used more frequently comparing to its synthesized chemical analog, pyrethrin, used in the present study.
Apart from fish, pesticides could also have a negative impact on biofilter bacteria in aquaponics setup (Goddek et al., 2015). Biofilter is a necessary element in every aquaponics system, receiving effluents from the fish tank, while bacteria populating filters are essential for the nitrifying process in the water. Tank effluents in aquaculture contain dissolved nutrients, mainly nitrogen and phosphorus, various inorganic and organic compounds, and suspended solids.
These components originate from uneaten feed and metabolic wastes from the fish. The main metabolic product of fish is ammonia, a toxic molecule that induces several negative effects on the same fish that excrete it (Eddy & Williams, 1987). In the first place, high environmental ammonia impairs ammonia excretion of fish or even causes a net uptake of ammonia from the water, which leads to deleterious effects in fish (Randall & Tsui, 2002).
The process of converting toxic ammonia to nitrates could be completed in one or two-step process, depending on microorganisms inhabiting the biofilter. In the two-step process, the first step is converting ammonia to nitrite by ammonia-oxidizing bacteria (AOB; e.g. Nitrosomonas sp.) and archaea (AOA; e.g. Thaumarchaeota sp.), while the second step is converting nitrite to nitrate by nitrite-oxidizing bacteria (NOB; e.g. Nitrobacter sp.).
This two-step process can also be completed by a single microorganism (e.g. Nitrospira sp.) in complete ammonia oxidation (comammox), as recently discovered (van Kessel et al., 2015). Similar to ammonia, nitrite is also toxic to fish, but to a lesser extent (Wuertz et al., 2013).
The described processes result in a constant reduction of ammonia levels in water and less toxic effects on fish (Effendi et al., 2017). Bacteria in biofilters are usually from two distinctive groups: autotrophic nitrifiers (responsible for oxidizing ammonia to nitrate), and heterotrophs (degrade organic matter using available oxygen in the water) (Blancheton et al., 2013).
It is also known that pesticides application have to stimulate (an increase of growth), negative (decrease of growth) or neutral effect on soil nitrifying microorganisms (Ahmed et al., 1998; Lo, 2010), but no research up to date was conducted for biofilters which could be used in the aquaponics.
The aim of the study was to apply synthetic and botanical insecticides to water containing working biofilters and to test biofilter efficiency in amelioration of ammonia stress by measuring concentrations of ammonia, nitrites, and nitrates in the water.
The present study was designed as a very possible scenario in aquaponics production as we complied with all the instructions for the application of insecticides at plants, and assuming 10% from the total application as dissipation and phloem transport. To authors’ knowledge, there are no published studies concerning the number of insecticides ending up in the aquaponics RAS system after application and little available data in hydroponics.
In one published study 7 various synthetic pesticides (organochlorins and pyrethroids) were applied to hydroponics cultivation of Gerbera flowers (Gerbera jamesonii) in the greenhouse (Hatzilazarou et al., 2004). 24 hours after application, concentrations in the water drain in the hydroponics system ranged from 0.5 to 18 μg L-1 (Hatzilazarou et al., 2004), which is in line with the nominal concentration applied to water in the present study. Moreover, concentrations applied in the present study were below 96 h LC50 for several fish species found in the literature.
As shown in Table 2, 96 h LC50 concentrations are varying considerably. Those variations are depending on a number of factors, such as: exposed species, insecticide formulation, fish age, size, body mass, and environmental conditions.
In any case, concentrations of insecticides used in the present study were not supposed to cause mortality to any of the fish species reared in an aquaponics system, not even to fish from the Salmonidae family, which are frequently highlighted as more sensitive to pesticides compared to other freshwater fish families (Macek & McAllister, 1970).
From the start of the experiment, ammonia concentrations in the water were reduced to very low values in 13 hours, confirming the efficiency of the first step of nitrification in biofilters. The same was not true for the second step of nitrification, which took 21 h for obtaining characteristic rhythm and therefore, efficiency.
Lower concentrations of nitrite-N were found after 5 h, which could be explained by lower concentrations at the beginning and lower efficiency of AOB/AOA in AZA group. This lag in the efficiency of NOB is quite common in newly-established RAS systems (Krüner & Rosenthal, 1983), and could also be explained by the partial change of the water prior to the beginning of the experiment.
The concentrations of NO2-N were not significantly higher compared to other groups, due to the large variability between groups, but the rising trend of NO2-N in AZA group is pointing to a high impact on the second step of nitrification in the aquaponics. The reason for intra-group variability and growing difference in standard deviation over time is probably due to the change of bacterial communities.
On the other hand, it is now known that two previously mentioned pivotal bacterial species (Nitrosomonas spp., as AOB and Nitrobacter spp., as NOB) are usually in very low quantities in biofilters and that AOA and comammox (Nitrospira spp.) are bacteria that are driving nitrifying process (Bartelme et al., 2017).
This means that the effect of AZA could have either a direct impact on the nitrifying bacterial community or, indirect, by optimizing the environment for other types of bacteria, presumably heterotrophic, that could populate biofilter and thrive as better competitors in the new conditions (Blancheton et al. 2013).
In any case, applying AZA to aquaponics is leading to higher concentrations of ammonia and nitrites which could eventually harm the fish or even lead to mortality. In the practical guideline for small-
scale aquaponics, the recommendation is given for TAN concentration in the aquaponics to stay <2 mg L-1 for Nile tilapia (Oreochromis niloticus), <1 mg L-1 for the majority of other fish species, and <0.5 mg L-1 for rainbow trout (Oncorhynchus mykiss) (Somerville et al., 2014). The toxicity of ammonia strongly depends on the pH levels and temperature of the water, as a toxic, unionized fraction (NH3-N) prevails at high pH levels and at high water temperature (Randall & Tsui, 2002).
In the exposure tests, it was confirmed that concentration of 2 mg L-1 did not cause mortality to the Nile tilapia, but some minor histopathological alterations in gills and liver were noted (Benli et al., 2008), while for the blue tilapia (Oreochromis aureus) there was no change in growth rate, feed conversion ratio and mortality in fish reared at 2.5 mg L-1 TAN during 53 days (Hargreaves & Kucuk, 2001).
As mentioned earlier, nitrites are less toxic compared to ammonia, and recommendations are that concentrations below 1 mg L-1 should be maintained in aquaponics (Somerville et al., 2014). The 96h LC50 of nitrite-N for Nile tilapia is determined to be 81 mg L-1 for small fish and 8 mg L-1 for large fish (Atwood et al., 2001), although lethal concentrations are strongly dependent on the chloride levels in the water (Yanbo et al., 2006).
Effect of AZA on NOB/comammox in the present study led to fast and steady growth of nitrites-N concentrations in the water, reaching the value of 3 mg L-1 after only 53 hours in the study. This represents a considerable difference in AZA compared to other three groups and an increase from 91 to 143 % after 45 h and from 57 to 75 % after 53 h.
Mechanism of toxicity differ for two mentioned molecules, as ammonia is brain toxicant in fish (Randall & Tsui, 2002), while nitrites are causing presence of methemoglobin and as a consequence reducing the concentrations of hemoglobin in the blood (Yildiz et al., 2006).
On the other hand, nitrates are considered relatively safe for fish, and less toxic comparing to ammonia and nitrites. 96h LC50 concentrations for NO3-N are very high and ranging from 191 to 2400 mg L-1, depending on fish species (Camargo et al., 2005). Nitrates are easily taken by plants in the aquaponics, and the concentrations in the water would be lower if the present trial was done in full aquaponics setup (including plants).
Plants can absorb ammonia from the water in the aquaponics, although to less extent comparing to nitrates, since AOB and NOB are attached to the root surface of the plants (Hu et al., 2015). Even though the intention of the study was to use CHL as a positive control, as it is proven to have an effect on soil bacteria (Lo, 2010), the same was not noted in the present study, probably due to the different environment used (Stark & Firestone, 1995), or due to the low concentration of pesticide in the water.
In conclusion, it was shown that applying AZA in the aquaponics setup is having a negative impact to concentrations of nitrogen compounds in both steps of nitrification. Therefore, caution has to be taken when this pesticide is applied. Other two pesticides did not show statistically significant difference from the control in any time point during the sampling.
The major point of concern in aquaponics is lack of toxicological studies of three toxic molecules combined (ammonia, nitrites and pesticide), as fish can struggle in the presence of multiple toxicants in the environment. The obtained results should be used as a starting point in other research of effects of pesticides in aquaponics, since there is a lack of available literature published.
Source: Rašković, B., Dvořák, P., & Mráz, J. (2021). Effects of Biodegradable Insecticides on Biofilter Bacteria: Implications for Aquaponics. Turk. J. Fish. & Aquat. Sci, 21(4), 169-177.
Useful Article: Evaluation Of The Environmental Impact Of An Aquaponics System