Assessment of land-based pollution problems in Kenyan marine environments to facilitate adaptive management of coral reef systems

Coral reefs are sensitive to environmental perturbations, and an unprecedented decline in corals has been reported globally as a result of increasing global and local stressors including excessive input of anthropogenic nutrients. This study investigated the effect of land-based sources of nutrients (N and P) associated with sewage, on ocean water quality and the health of corals in Mombasa Marine National Park and Reserve in Kenya to inform integrated coastal zone management and ocean governance. A year-long study was conducted to determine water quality according to protocols described in Grasshoff et al . (2007). Coral health status was also monitored using Underwater Visual Census (UVC) to record coral reef ecological parameters. The study area’s temperature, salinity, pH and dissolved oxygen were within the recommended standards for healthy coral reefs. The study indicated that land-based nutrients, Chlorophyll-a ( Chl a ) and total suspended solids (TSS), are the key factors affecting corals and could be the reason for the observed coral health, which ranged from fairly healthy to unhealthy. On average, nutrient concentrations were higher than recommended to maintain at least 50% coral coverage. Ammonia was the dominant form of nitrogen ranging from 0.105 to 0.4130 mg/l, while nitrate concentrations were 0.0348-0.0468 mg/l, indicating the possibility of blooming algal species in the area. Total suspended solids were above the recommended values, ranging between 33.5 and 79.3mg/l and Chl a 0.7114 and 1.58 μg/l. The study concluded that land-based nutrient load influenced coral reef health during the study period. It recommends that land-based pollution needs to be addressed as part of a holistic, integrated coastal zone management approach supporting practical, sustainable and legal management of nutrient discharge into the marine environment to preserve the water quality of Mombasa Marine National Park and Reserve.

Approximately 80% of marine pollution originates from land-based sources that reach coastal waters via diffuse run-off, direct waste deposit, and atmospheric fallout (Daoji and Daler, 2004;McIntyre, 1990). Most eutrophication and organic pollution in coastal regions in the world is linked to the discharge of sewage (defined as a cocktail of waste from food preparation, dishwashing, garbage-grinding, toilets, baths, showers and sinks) effluent and dumping of sewage sludge (Okuku et al., 2011). Moreover, an increase in food production due to population growth has resulted in a concentration of these nutrients on land as well as changes in the global hydrological cycle, doubling the rate at which biologically available nitrogen and phosphorus enter the marine ecosystems (Galloway et al., 2004;Seitzinger et al., 2005). This is compounded by poorly developed sewage waste management infrastructure and inadequate domestic waste management facilities, with much of the effluent from industries and tourist hotels emitted directly into the coastal waters (Okuku et al., 2011).
Nutrients such as nitrogen (N) and phosphorous (P) are essential for supporting the productivity of coral reef ecosystems. However excessive enrichment of marine and coastal waters by these nutrients associated with anthropogenic activities can cause deleterious effects, making them one of the major threats facing coral reefs globally (Lapointe et al., 2010). By stressing coral physiology and functioning through increased water temperature, increased algae cover, and seaweed development that competes with the corals for space and light, excess nutrients can cause coral ecosystems to collapse (Smith et al. 1981). A reduction in light penetration results in reduced zooxanthellae photosynthesis, which reduces coral productivity (DeGeorges et al., 2010). Enhanced macro-algae growth can smother and kill corals (Littler et al., 2006), negatively affecting them by shading/ overtopping, reducing water exchange, and causing mechanical abrasion or chemical disturbance.
Besides enhancing the susceptibility of coral reefs to bleaching (Mangi et al., 2007), algae can also release toxins, deplete oxygen and increase the risk of bacterial and fungal infections contributing to the spread of coral diseases (D'Angelo and Wiedenmann, 2014).
Lower calcification rates, reduced reproductive success, altered skeletal density, and linear extension in response to heat and light stress are some of the observed reactions of corals to elevated nutrient levels. Studies have shown that anthropogenic nutrient enrichment of reef waters contributes to the deterioration of coral reefs close to urbanised and heavily populated areas. (Fabricius et al., 2003;Wagner et al., 2010;Wooldridge, 2009).
To conserve marine ecosystems from land-based pollution, governance institutions should develop policies to monitor and regulate the quality and quantity of nutrients released into coastal areas. There is limited data on the link between coral health and nutrient load in the WIO. At the same time, many global marine regulations do not integrate land-based controls, making them prone to failure (Carlson et al., 2019). Marine reserves tend to be static (e.g., hotspots for marine biodiversity) rather than representing the time-variant dynamics that define land-sea processes, such as contaminant flows (Stoms et al., 2005, Arias-González et al., 2017. These issues are among several that have hindered the design and implementation of the regulation of discharges and management of wastes from urban developments and agricultural inputs in reef catchments. This highlights the need to couple land-sea planning while recognising the complexities associated with executing ridge-to-reef conservation approaches (Carlson et al., 2019;Arias-González et al., 2017;Stoms et al., 2005).
This study provides information on the water quality status in coral reef ecosystems. Nutrient quality and quantity and coral reef health were assessed in the Mombasa Marine National Park and Reserve in Kenya. The study also aimed to establish the relationship between water quality, in terms of nutrient concentrations, and coral reef health. This is useful for managers and decision-makers in formulating holistic and best practices in management and governance for the conservation and sustainability of coral reefs.  (Fig. 1). The Park measures 10 km 2 and is open to public recreation, but extractive uses are prohibited ("no-take" zone). The Reserve measures 200 km 2 , allowing public access and controlled extractive use of resources. It has coral reefs in its waters and encloses part of the lagoon, back reef and reef crest habitats of the Bamburi-Nyali fringing reef. The MPA has other critical habitats -seagrass beds, sandy beaches and intertidal flats -that are an essential source of coastal livelihoods such as fishing and tourism. The MMNP&R is adjacent to Mombasa city, making it vulnerable to numerous threats (Tuda et al., 2007).

Water quality
The year-long study was carried out between September 2017 and August 2018 to measure seasonal variation in water quality and its potential impact on coral reef health. Samples were collected once a month on the first Tuesday/Wednesday of the month. Seasons were classified as short rains (September to November), dry (December to February), long rains (March to May), and cold ( June to August).
Seven stations were selected and clustered into three distinct groups for water sample collection and coral health monitoring (  The study examined the potential effects of nutrient pollution on corals by carrying out assessments of coral reef health and water quality in terms of nutrient (N and P) quality and quantity, usually measured as Chl a in the water column, which is a robust indicator of increased nitrification (Brodie et al., 2007;Furnas et al., 2005). Data collection were done both

Trophic state index (TSI) classification
The trophic state index (TSI), developed by Carlson (1977), is used to measure the water quality of water bodies. It has three states: oligotrophic (low primary productivity due to nutrient deficiency);  With TP and Chlorophyll-a measured in micrograms per litre (µg/L), and SD transparency in meters.

Assumptions
The study was carried out to investigate the impact land-based nutrient loads would have on marine systems, with corals being used as indicator species.
While there are many factors such as human activities within the study area that impact coral reef health, the focus of this study was on nutrient concentrations.
Temperature was also considered as it is a standard parameter when analysing water quality.

Results and discussions
Water quality The physico-chemical properties of the sampling sites showed that the average monthly temperature ranged from 22.7 ± 10.0 to 28.2 ± 0.1 ᵒC observed at Nyali and Mtwapa Prison. Studies conducted by Hoegh-Guldberg (1999) in similar coral ecosystems show similarities to the study area and describe these temperature ranges as ideal for coral's optimal growth (Hoegh-Guldberg and Bruno, 2010). Typically, the temperature range for the formation of corals is 18 -36 °C, with the optimal temperature between 22° and 28 °C (Wilkinson, 1999;Hubbard, 1997 (Guan et al., 2015), demonstrating that the salinity in the study area is ideal for the growth and development of corals and is therefore not a limiting factor for coral health.
Dissolved oxygen (DO) showed a wide variation ranging from 6.0±2.1 mg/l to 15.7± 5.9 mg/l. The highest value was recorded in Ras Iwatine (MMNP&R) and the lowest at Mtwapa Mouth (Creek). Table 1  Other studies found that excessive amounts of nitrogen and phosphorous have been linked with the reduction of DO in marine water systems to the point of causing hypoxia (Dodds, 2006). However, all stations had levels higher than the recommended standard of 4 mg/l (Shanmugam et al., 2007) (Table   2). This implies that DO did not contribute to the deterioration of coral health in MMNP&R during the study period (Table 2). Total suspended solids (TSS) affect coral reef growth (Parwati et al., 2014). Effects of sedimentation on the coral reef are a significant factor that results in the smothering and death of corals during the recruitment process (Fabricius et al., 2003). The results for TSS ranged from 33.7 mg/l in the Mtwapa Mouth to 79.3 mg/l at Coral Garden, with an average of 55.5 mg/l across all seven stations. As shown in Table 2, the TSS across all stations is higher than the recommended standards of <25 mg/l (Shanmugam et al., 2007). These values suggest that the high TSS levels in the study area could contribute to the deterioration of coral health during the study period. Proper sewage treatment to remove suspended solids, organic matter and nutrients is necessary before the effluent is discharged into aquatic bodies (Rono, 2017).
The highest levels were recorded between April and

Phosphates
Phosphorus is a limiting nutrient, particularly in tropical and subtropical estuarine and marine systems (Caffery et al., 2007). Phosphates in the water samples Prison, which showed a spike in October 2017 (Fig. 6).
The analysis of variance confirms that there are no significant differences in phosphate concentrations (mg/l).

Chlorophyll-a
Chlorophyll-a measures the green pigments in photosynthesising algae in the marine environment. The was analysed (Fig. 7). April and August 2018 had the highest concentrations of Chl-a across all sampling stations (Fig. 7). Statistical analysis showed that, based on      Kinjo (2017) presented a set of nutrient concentrations needed to maintain at least 50% coral reef coverage in a given area (Table 3). While the average amount of phosphates (0.021 mg/l) and nitrates + nitrites (0.0408 mg/l) were below the coastal water standards (Table 2) ratios (Rosset et al., 2017). Nutrient enrichment plays    Table 3. Environmental guidelines for coral reef conservation for acceptable levels of P and N to maintain 50 % of coral coverage as highlighted by Kinjo (2017). The observed averages in the study areas and their molar ratios.

Water quality parameters
Water quality is required to maintain 50% of coral coverage to 7.2:1 (Smith et al., 1981;Crossland et al., 1984;Furnas et al., 1995). The results recorded in this study showed an N:P ratio of 3:1, which is a lower ratio than those recommended by the previous studies. This lower ratio suggests that there is a higher concentration of phosphorous than nitrogen in the study area, which would negatively impact the health of corals. This is further supported by a study by Larned (1998) that concluded that higher concentrations of phosphorus, rather than nitrogen, are the primary limiting nutrient to coral and macroalgae productivity.

Carlson's TSI classification
According to Carlson (1977), the changes from oligotrophic to eutrophic do not occur at sharply defined places or at the same location or rate. This implies that water bodies can be considered oligotrophic by one criterion and eutrophic by another. This is evident in the results from the current study in Table 4

Management of nutrient discharges
Land-based pollution needs to be addressed as part of a holistic, integrated coastal zone management approach supporting practical, sustainable and legal management of nutrient discharge into the marine environment to conserve corals. Efforts have been made to address land-based activities by formulating Strategic Action Plans (SAPs) to address the challenges of increased coastal water pollution in the Western Indian Ocean region (Pole et al., 2016). Several methods to reduce nutrient discharge have also been developed around the world that can be adopted in Kenya

Conclusions and recommendations
The study found that the land-based nutrient load would influence coral reef health during the study