4.1 Agricultural intensification in Lake Naivasha catchment
In the 20 years between 1989 and 2019, the extent of grassland in the lower part of the catchment, and cropland and human population in the upper catchment of L. Naivasha has increased 26,43. The catchment is known for its diverse agriculture, including crop cultivation, livestock rearing, and horticulture. This has implications for the potential export of both nutrients and pesticides from land to water, and consequences for the ecosystem health of the rivers and lake. Increased population density in the upper catchment to above 600 persons per km2 is associated with smaller division of land holdings, and the need for greater intensification involving both nutrient and pesticide additions 3. Smallholder farmers in the L. Naivasha catchment grow a variety of crops including maize, beans, wheat, and vegetables, while livestock reared in the catchment include cattle, sheep, and goats for both commercial and subsistence livelihoods. Additionally, the thriving horticulture in the 50 km2 around the lake, especially floriculture, increases potential application of N and P fertilisers44. While the model used in this study indicates that extensive agricultural systems had a higher probability to export nutrients and pesticides than intensive agricultural systems, this also depends on the nature of the land management, and the spatial pattern of nutrients and pesticides detected from field sampling.
4.2 Nutrients and pesticides emission in the catchment
Maximum nutrient concentrations in L. Naivasha catchment found at K1 (R. Karati Highway Bridge) could be associated with the number of farms close by, that likely provide point sources from farm drains. Site K1 site has been previously shown to have high concentrations of heavy metals as well as nutrients 45,46. Although Everard et al.47 postulated that the geological difference between the R. Karati sub-catchment and the other river sub-catchments could explain the high chemical contamination at K1, the high concentrations of TP (1551 ± 179 ug TP l− 1) found at the site suggests point-source pollution.
The high discharges in R. Malewa compared with the other rivers in the L. Naivasha catchment results in high loads of nutrients from the R. Malewa catchment. However, the vast geographical area of the R. Malewa catchment, and the relatively lower nutrients concentrations compared with R. Karati, makes R. Karati a bigger contributor to nutrients yield into the lake per unit area. The R. Karati had the highest concentrations of total nitrogen (TN) of the rivers in the L. Naivasha catchment, while the lake itself had the lowest TN concentrations. This trend has been recorded in other catchments where the draining rivers have higher concentrations than the receiving body48 owing to dilution in recipient water bodies 49, as well as sedimentation 50.
Yongo et. al.51 attributed the transition of L. Naivasha from eutrophic to hypereutrophic status to increased nutrient inputs. The transition of, especially TP, between the lower reaches of the influent rivers, the river mouth and the open lake suggests settlement in the lake predicted by traditional loading models 50. It is also apparent that compared with measurements made in 2002 by Kitaka et al. 52, concentrations of P in the lake have increased approximately three-fold. In this study we report the TN:TP ratios in the rivers, with results indicating the potential of some of the sites being P-limited. There are nutrient interactions and transformations within the river, owing to the biophysical (including biofilms) and hydrological regime that can make interpreting TN:TP ratios and P limitation difficult 53,54. It is clear that there is a need for long-term monitoring and integration of multiple data sources to better quantify and understand the movement of nutrient from the catchment to the lake 55,56.
The pesticide results suggest continued use of DDT, despite a ban on use except by public health officials 57,58. This study recorded a hundred-fold higher concentration compared with the US EPA water quality standards. That the ∑DDT was highest in the lake samples suggests accumulation and DDT resuspension due to bioturbation and wave action that can re-release DDT into the water column 59. Moreover, the long environmental half-life of DDT 60 could mean that DDT flushed from the rivers, is trapped within the lake at higher concentrations 61. However, high DDT:DDE ratios across the catchment indicate recent and high inputs from the flower farms that border the lake. That ∑HCH was highest in the lake suggests continued application of technical lindane around the lake as reported by Onyango et al. 57,62, but up to seven times the values recorded in 2011 by Njogu 63. The concentrations of ∑Cyclodienes were highest in the lake sites, the Gilgil river, and lower reaches of R. Malewa. The values recorded in this study were higher compared with the concentrations recorded in 2002 by Gitahi et al. 64, suggesting continuous increase in use of cyclodienes based pesticides. The distribution of the cyclodienes within the lake showed high concentrations within the deeper parts of the lake with relatively lower anthropogenic disturbance (M3 – Hippo point), as documented by Ndungu et al. 23 and Outa et al. (2014).
4.3 Relationship between intensification and nutrients and pesticides emission
Agricultural intensification can have profound effects on aquatic ecosystems, leading to significant changes in water quality, biodiversity, and overall ecological balance. This study indicates the potential that the intensification in L. Naivasha catchment is contributing to nutrient enrichment in the rivers and eutrophication in the lake. Having recorded higher nutrients and pesticides concentrations compared with previous studies such as Kaoga et al. 65 with data from 2010, Otieno et al. 66 with data from 2011, and Onyango et al. 57 with data in 2012, coupled with increased land use on cropland, the potential of excessive nutrient and pesticides runoff or leaching from agricultural fields into nearby water bodies is inevitable.
High pesticide residues associated with forest areas are comparable with the concentration of DDT reported by Onyango et al. 57 within the upper reaches of the R. Malewa sub-catchment. This is somewhat counter-intuitive, but could reflect that forested areas in the L. Naivasha catchment are associated with fragmented, and small scale, intensified agriculture involving pesticide use for crops such as maize, beans, wheat, and vegetables which are prone to insect and fungal attacks in the area. While this study argues that pesticides and nutrients are used together in intensification, the study found no significant relationship between pesticide and nutrient concentrations. Both the temporal intensity of sampling and differences in modes of action between nutrients and pesticides 67,68 and the pathways of transfer to aquatic sources 69,70 could account for this.
4.4 Management of agricultural intensification
Agricultural intensification has undeniable importance for meeting global food demand, but it also poses significant impact on aquatic ecosystems. The impacts of intensified agriculture on water bodies are multifaceted, encompassing nutrient enrichment, pesticide contamination, sedimentation, hydrological alterations, and biodiversity loss 71. Addressing these impacts requires a comprehensive and integrated approach that combines sustainable agricultural practices, land use management, and effective monitoring and regulation 72,73. Only through such measures can the adverse effects of agricultural intensification on aquatic ecosystems be mitigated, ensuring the long-term health and sustainability of water resources and the biodiversity they support.
This study applied a risk-based approach to guide better catchment management. Notably the approach involved identifying and assessing potential risks to the L. Naivasha ecosystem and prioritizing management actions based on the level of risk posed. However, the study makes reference to only one stressor – water quality as a result of nutrients and pesticides emissions. As such, the approach applied did not consider other stressors and threats to the catchment, such as habitat degradation, and water abstraction. Nonetheless, the study has identified needs for more focussed assessment of the nutrient and pesticide combined risk to water quality. As a basis, the study has mapped out the lower reaches of the catchment to require more management attention, as there is cumulative contamination from upstream, with higher volumes of flow, amidst increased agricultural activities, especially from floriculture. The documented LULC fragmentation is poised to continue, on one hand because of population increase demanding land fragmentation and on another, the pressure to feed the growing population, while enhancing incomes. As a management tool, a risk map presents catchment managers with an entry point on water quality management, and a guide for monitoring. This allows for the allocation of limited resources and implementation of targeted measures to areas of highest risk, maximizing the efficiency and effectiveness of management efforts. Additionally, a risk-based approach promotes adaptive management, as ongoing monitoring and evaluation help to refine strategies and address emerging risks. By adopting a risk-based approach, river catchment management can proactively address threats, protect water quality, preserve biodiversity, and ensure the sustainable use of water resources for present and future generations 74,75.
The study identified recent and continued emission of pollutants from agriculture to the surface waters of the L. Naivasha catchment as a management gap. This includes potential impact from banned pesticides used across the catchment, land use changes with complementary intensification practices, and potentially high to very high risk of combined nutrients and pesticide chemical pollution of surface waters. These risks occur across many agricultural catchments in sub-Sahara Africa (SSA) 76–79. Standards for managing aquatic resources are highlighted in the Africa Water Vision 2025 80 advocating for a revision of water regulations and laws to give attention to water quality management. Based on the findings in this study, the revision of the Africa Water Vision 2025 needs to take into consideration water quality standards that incorporate combined nutrients and pesticide use and their emissions from land to water.
Kenya’s regulations on water quality standards81 does not consider the potential of combined effects of nutrients and pesticide, neither does it consider monitoring, review, and policing framework for banned substances in surface waters. Moreover, the standards do not have provisions to monitor effluents associated with banned pesticides. This is a clear indication of a mismatch between regulation and enforcement, that requires the responsible agencies such as the Kenyan Water Resources Authority (WRA), National Environment Management Authority (NEMA), WASREB, and the Kenya Bureau of Standards (KEBS) to actively revise the water quality standards, enforce the management of the standards, and invest in policy and institutional reforms to address management of agricultural intensification.
Policy and institutional reforms contributing to development of regulations that consider the increased risks to surface water and promotes enforcement for compliance to the regulations can provide considerable benefits for sub-Saharan Africa agricultural management. Further, the partnerships among national and subnational governments and the private sector can promote coherence of regulations across jurisdictions, and coordination among strategic water quality management and policy authorities. Catchment management partnerships – such as Imarisha in Naivasha 82, - as a self-organized community for water resources management 83, have an opportunity for inclusive and integrated management. At the same time, a monitoring regime such as the one proposed in the L. Naivasha Basin Integrated Management Plan for 2012 -202284, still require availability of consistent and continuous water quality data, to track and document progress in the development of water resources regulations and policy85. The water quality guidelines in Kenya provides for frequency of monitoring, single chemical and biological standards, methodologies for collection and analysis of samples, and a reporting framework 41. However, the standards do not take into consideration combined contamination. Moreover, strategies developed are still seen to enhance marginalization of local stakeholders, reducing the potential for ownership, and therefore enforcement of any pollution reduction measures86. The possible solution to bridging the gap between nutrients and pesticides emissions, with agricultural intensification and land use changes may require continuous biotic monitoring of lakes and rivers.. Their presence, abundance, and diversity can serve as indicators of ecosystem health and water quality conditions. Numerous studies have demonstrated the effectiveness of macroinvertebrates as bioindicators in assessing water quality and ecological integrity 87–90. The use of metrics, such as the Biological Monitoring Working Party (BMWP) score, the Family Biotic Index (FBI), the Shannon-Wiener Index and Species at Risk (SPEAR), derived from macroinvertebrate communities, allows for the evaluation and comparison of water quality across different sites and over time 91. These approaches would provide valuable information for water resource managers and decision-makers in designing effective conservation and restoration strategies to maintain and improve the quality of aquatic ecosystems.
The study shows that intensification, accompanied by increased application of nutrients and pesticides 4, needs sustained advisory capacity in the use of agricultural inputs 92. However, the capacity and resources to manage the advisory services are still underdeveloped in Kenya, as they are in many sub-Saharan African countries. Inadequate resources to manage advisory services is exacerbated by inadequate standardized monitoring methodologies. The United Nations Environment Programme in its Progress on Ambient Water Quality update 93, report findings from methodological considerations for monitoring water quality. However, the report recommends that the monitoring of ambient water quality should use national and/or subnational water quality standards. In many SSA countries, the standards are not comprehensive, a situation that reinforces the need for integrated review for water pollution management at national level.
An integrated review of standards would contribute to achieving and compliance with the Sustainable Development Goals (SDGs) including: SDG 2 through promotion of sustainable agriculture practices, SDG 14 through reduction of risks to aquatic biota, SDG 3 through reducing the potential of health complications from exposure to contaminants in the aquatic systems, SDG 6 through availing better water quality and promoting sanitation, and SDG 17 through promoting inclusive partnerships for monitoring and review. Further SDG indicator 6.3.2, provides a mechanism for determining whether, and to which extent, water quality management is successful, with a target to increase the proportion of water bodies with good water quality 93. The findings from this study, emphasizes the need for water quality managers to consider combined contamination from nutrients and pesticides, and how that is monitored.