Introduction
Seawater colour is largely controlled by suspended particles that scatter and absorb light, and discolouration is commonly linked to rapid phytoplankton growth under favourable hydrographic conditions (Paerl, Reference Paerl1988; D’Silva et al., Reference D’Silva, Anil, Naik and D’costa2012). In Indian coastal and estuarine waters, such events typically occur during the southwest monsoon (SWM) and northeast monsoon (NEM) due to increased nutrient inputs from river discharge and coastal processes (Nair et al., Reference Nair, Devassy, Madhupratap, Vollenweider, Marchetti and Viviani1992; Parab et al., Reference Parab, Prabhu Matondkar, Gomes and Goes2006; Oyeku and Mandal, Reference Oyeku and Mandal2021) (Table 1). In Indian coastal waters, diatoms generally dominate bloom events along the east coast, while dinoflagellates are more common along the west coast (D’Silva et al., Reference D’Silva, Anil, Naik and D’costa2012; Jyothibabu et al., Reference Jyothibabu, Karnan and Jagadeesan2017). While most studies have focused on planktonic blooms in open and estuarine waters, bloom-forming assemblages can also develop within shallow intertidal environments, resulting in temporary discolouration of exposed sediments and overlying intertidal waters.
Summary of reported instances of water discolouration caused by plankton blooms in Indian estuaries and coastal waters
Table 1 Long description
The image A showing a locator map of India with the label “Goa” on the central west coast of India and the label “Arabian Sea” offshore. A dashed connector points from the Goa area toward the next map. The image B showing a satellite-style map labeled “Mandovi Estuary” and “Zuari Estuary,” with the label “central west coast of India.” A boxed area marks a smaller focus region and a dashed connector points to the next map. The image C showing a closer satellite-style map labeled “Mandovi Estuary,” “Caranzalem beach,” and “Dona Paula beach.” A scale bar labeled “1 km” appears near the bottom. A boxed area marks a smaller focus region and a dashed connector points to the next map. The image D showing a further zoomed satellite-style map labeled “Mandovi Estuary” and “Caranzalem beach.” A curved shoreline segment along Caranzalem beach is outlined, with four numbered markers labeled “1,” “2,” “3,” and “4” placed along the outlined shore segment. A scale bar labeled “500 m” appears near the lower left. The image E showing an aerial view with a single arrow marker and coordinate text: “15°27′40.3”N” and “73°48′02.9”E.” The image F showing an aerial view with a single arrow marker and coordinate text: “15°27′46.3”N” and “73°48′01.1”E.” The image G showing an aerial view with a single arrow marker and coordinate text: “15°27′50.8”N” and “73°48′14.5”E.” The image H showing an aerial view with a single arrow marker and coordinate text: “15°27′54.8”N” and “73°48′18.4”E.”.
In addition to planktonic blooms, microphytobenthic assemblages are widely reported along Indian coasts and estuaries (Mitbavkar and Anil, Reference Mitbavkar and Anil2002, Reference Mitbavkar and Anil2004; Sanilkumar et al., Reference Sanilkumar, S and Joseph2009, Reference Sanilkumar, Saramma, Joseph and Cahoon2011; Ingole et al., Reference Ingole, Gaonkar and Deshmukh2014; Krishnan et al., Reference Krishnan, Patil and Anil2022). Microphytobenthos play a significant ecological role as a primary live food source for the benthic fauna that dominate intertidal and subtidal habitats (Kuipers et al., Reference Kuipers, De Wilde and Creutzberg1981). Although detritus typically constitutes more than 70% of benthic food resources, live algal cells can serve as preferred food when blooms occur (Kuipers et al., Reference Kuipers, De Wilde and Creutzberg1981). Furthermore, microphytobenthos act as ‘ecosystem engineers’, contributing to sediment stabilization (de Brouwer et al., Reference de Brouwer, Wolfstein and Ruddy2005; Gerbersdorf et al., Reference Gerbersdorf, Bittner and Lubarsky2009). In shallow intertidal systems, sediment resuspension driven by tides and wave action can redistribute benthic microalgae into the overlying waters, thereby influencing both sediment and water colouration. Despite their importance, reports on benthic diatom blooms and associated discolouration of intertidal zones remain scarce. For instance, a bloom of Asterionella japonica along the Waltair coast of India caused greenish discolouration of seawater and the intertidal zone (Rao, Reference Rao1969). Similarly, eutrophication-driven blooms of macroalgae (Ulva spp.) on sandy shores of the central west coast of France produced large-scale ‘green tides’ (Quillien et al., Reference Quillien, Nordström and Bris2018). Recent unmanned aerial vehicle (UAV)- and image-based studies from the Guadalquivir Estuary (Spain) also reported visually distinct brown intertidal biofilms associated with dense microphytobenthic diatom assemblages on estuarine mudflats (Román et al., Reference Román, Oiry and Davies2024).
Typically, the colour of intertidal zones is attributed to the mineralogical composition of sand, shaped by local geology and coastal processes (Asensio-Montesinos et al., Reference Asensio-Montesinos, Pranzini and Martínez-martínez2020). In contrast, this study documents instances of temporary intertidal discolouration caused by microphytobenthic blooms and the accumulation of organic matter. Several hydrographic factors regulate the growth, productivity, and bloom formation of microphytobenthos, including temperature, light, nutrient availability, salinity, and turbidity (Barranguet et al., Reference Barranguet, Kromkamp and Peene1998). In the Indian seas, phytoplankton production has been reported to be primarily nutrient-controlled rather than light-limited, owing to their tropical location (Paul et al., Reference Paul, Ramaiah and Sardessai2008; Jyothibabu et al., Reference Jyothibabu, Arunpandi and Karnan2018). Accordingly, this study aimed to (a) quantitatively document and characterize changes in intertidal surface and overlying water colouration through field observations, pigment analysis, microscopic examinations, and hydrographic measurements; (b) identify the primary environmental and biological drivers (e.g., temperature, salinity, and nutrient concentrations) associated with discolouration events; and (c) evaluate the role of hydrographic conditions in facilitating or constraining the occurrence of phytobenthic blooms along the Dona Paula and Caranzalem shores during the SWM and NEM seasons.
Methods
Study area and sampling strategy
The present study was conducted along the intertidal zones of the Dona Paula shore and the Caranzalem–Miramar shore stretch (∼2.5 km) of the Mandovi–Zuari estuarine system on the central west coast of India (Goa) (Figure 1a–d). During the SWM, the estuary receives substantial freshwater input from the Mandovi and Zuari Rivers. Consequently, concentrations of nutrients such as nitrate, phosphate, and silicate increase notably due to river supply (Pednekar et al., Reference Pednekar, Matondkar and Gomes2011; Karnan et al., Reference Karnan, Sandhya, Gauns and Pratihary2021). Previous studies have reported that phytoplankton abundance and biomass decrease during the SWM compared to other seasons (Pednekar et al., Reference Pednekar, Matondkar and Gomes2011). Although phytoplankton blooms are seldom reported from the Mandovi Estuary, available studies emphasize the role of salinity, light, transparency, and nutrient availability in regulating phytoplankton dynamics (Devassy and Goes, Reference Devassy and Goes1988; Patil and Anil, Reference Patil and Anil2011; Pednekar et al., Reference Pednekar, Prabhu Matondkar and Kerkar2012; Ramakrishnan and Fernandes, Reference Ramakrishnan and Fernandes2022).
Map of the study area showing: (a) the Goa region along the central west coast of India, (b) the Mandovi and Zuari Estuaries, (c) Caranzalem and Dona Paula beaches within the Mandovi–Zuari estuarine system, and (d) the intertidal zone (highlighted in yellow polygon) along with stream inlets (indicated by red arrows) at Caranzalem. Panels (e–h) show Google Earth aerial views of the stream inlets connected to the Caranzalem intertidal zone.
Field observations were carried out between May 2023 and February 2024. Fortnightly observations were conducted across these zones, and when discolouration was detected, the observation frequency was increased to daily or alternative-day observations. These estuarine shores are characterized by a wide intertidal zone (∼150 m) resulting from their gentle slope, which exposes large areas during low tide (e.g., Caranzalem ∼0.34 km2).
The selected study areas also receive smaller stream inlets that discharge sewage and freshwater into the estuary (Figure 1d). Representative aerial views of the small stream inlets connected to the study areas have been provided (Figure 1e–h) for contextual reference. However, quantitative discharge measurements and seasonal sewage input assessments were not available during the present study. Intertidal observations were conducted during low tide, irrespective of the time of day, at frequencies ranging from daily to fortnightly, depending on bloom occurrence and visible discolouration intensity. During periods of visible discolouration or bloom incidence, observations were intensified to daily or alternate-day intervals. The study documented discolouration events from the onset of the SWM (June–September 2023) through the NEM (November–February), together with associated hydrographic conditions and potential biological causes. Discoloured water and intertidal areas were photographed systematically to enable comparison of discolouration patterns over time.
Sampling, observations, and measurements
In situ measurements and sample collection
Water temperature and salinity were measured in situ using a digital thermometer (Hanna Checktemp 1) and a salinity tester (Hanna HI98319), respectively. The photosynthetically available radiation (PAR) was monitored using a Quantum meter (MQ-500; Apogee). The water turbidity was measured using a Turbidimeter (TN-100; Thermo Scientific). All measurements were taken at the sampling site immediately during sample collection. The water and surface sediment samples were collected from where the discolouration of the intertidal zone and overlying water was observed. A 100 mL water sample was collected in a clean polyethylene bottle and stored in an icebox for the analysis of nutrients (nitrate, phosphate, and silicate) following standard spectrophotometric procedures (Grasshoff et al., Reference Grasshoff, Ehrhardt and Kremling1983). A separate 100 mL discoloured water sample was collected in a clean, opaque polyethylene bottle and kept in ice box for chlorophyll-a estimation. For plankton analysis, an additional 100 mL water sample was collected in an opaque polyethylene bottle and preserved with 3% Lugol’s iodine. Surface sediment samples representing the upper visible layer (∼0.5 cm) from visibly discoloured intertidal areas were collected using a spatula into clean polyethylene bottles, stored in an ice box, and transported to the laboratory for microscopic observation and identification of the dominant organisms associated with the discolouration events.
Laboratory measurements and procedures
The water sample for chlorophyll-a analysis was filtered immediately upon arrival at the laboratory (within 30 minutes of collection) using GF/F filter paper (0.7 µm pore size and 47 mm diameter). The filters were then stored at −80°C until the subsequent determination of phytoplankton biomass (chlorophyll-a). Chlorophyll-a was measured using the acetone extraction method (90%; Grasshoff et al., Reference Grasshoff, Ehrhardt and Kremling1983) and quantified with a fluorometer (Turner Designs). One of the limitations of the present study is that sediment chlorophyll-a was not quantified, although such measurements would provide a more quantitative estimate of benthic algal biomass associated with discolouration events. Instead, qualitative microscopic examination of surface sediments was conducted to identify the dominant microphytobenthic taxa associated with the discolouration events. Future monitoring efforts should include chlorophyll-a and other pigment measurements from different sediment layers to better quantify benthic microalgal biomass. Chlorophyll-a concentrations in the overlying waters were considered an indirect proxy for phytobenthic algal biomass suspended or released during sediment resuspension events.
For algal cell counting and identification, the Lugol-fixed sample was concentrated by siphoning it through a 10 µm bolting-silk mesh attached to the end of the siphoning tube. The concentrated sample was then examined under a Nikon ECLIPSE 80i microscope using a Sedgewick-Rafter counting chamber to estimate cell abundance. Algal identification was carried out primarily following Tomas (Reference Tomas1997) and supplemented with additional taxonomic references for diatoms and euglenophytes (Kim et al., Reference Kim, Boo and Zakryś1998; Taylor et al., Reference Taylor, Harding and Archibald2007). Additionally, a small amount of wet sediment was placed on a microscope slide, mixed with a few drops of filtered seawater, and live algal cells were observed and imaged using Nikon SMZ18 and Nikon ECLIPSE 80i microscopes at various magnifications.
Colour analysis of intertidal surface imagery
For standardized comparison, intertidal zone surface colouration was quantified from field photographs using a custom Python workflow developed with the OpenCV and Matplotlib libraries (SM-1). For each photograph, a region of interest (ROI) of appropriate dimensions was manually selected to represent the dominant surface condition (e.g., phytobenthic mats or bloom-induced discolouration) while avoiding background structures, areas beyond the water-reached zone, shadows, and other non-biological artefacts. The mean red-green-blue (RGB) value of each ROI was computed by averaging all pixels within the selected region and subsequently converted to the hue-saturation-value (HSV) colour space using matplotlib.colors.rgb_to_hsv. Hue values were expressed in degrees (0–360°), where 0° denotes red, 120° green, and 240° blue, whereas saturation and value ranged from 0 to 1. Hexadecimal (HEX) colour codes corresponding to the mean RGB values were additionally generated. For descriptive standardization, the extracted RGB and HEX values were compared against the 147 CSS3/Web colour references implemented in the Webcolors library, and the nearest colour category was assigned using Euclidean-distance matching in RGB space. In addition, hue-based descriptive categories were assigned from HSV hue-angle intervals to improve differentiation among visually similar colour classes. For visualization, the workflow generated a three-panel composite image comprising the selected ROI, a mean RGB colour swatch annotated with RGB and HEX values and the nearest CSS3 colour name, and a pure-hue HSV swatch generated from the extracted hue component (H) at maximum saturation and value (S = 1, V = 1), annotated with H, S, V, HEX values, and the corresponding hue-category label. These standardized colour descriptors and visual outputs were included to facilitate consistent comparison and reproducibility in future studies.
Results and discussion
Seasonal hydrographic variability
Plankton blooms are well-known drivers of water discolouration in the northern Indian Ocean, with numerous events reported from coastal and estuarine waters of the Arabian Sea and Bay of Bengal (Table 1). These blooms typically occur during the pre- and post-monsoon periods and are dominated by dinoflagellates, cyanobacteria, and diatoms, each producing characteristic colour signatures. While such studies primarily focus on pelagic phytoplankton blooms, they highlight the region’s susceptibility to rapid biomass accumulation under favourable hydrographic conditions. In contrast, the present study documents a lesser-reported phenomenon: short-term discolouration of intertidal zones associated with phytobenthic blooms and organic matter deposition. Distinct seasonal variability in environmental parameters was observed during the study period (Figure 2), which corresponded with shifts in dominant bloom-forming taxa across different monsoon phases.
Variations in key environmental parameters and phytoplankton abundance at the study site across three sampling periods: Late-May (greenish beach; see Figure 3), Mid-August (golden beach; see Figure 4), and Late-December (bright greenish beach; see Figure 5). Measured parameters include: (a) temperature, (b) salinity, (c) photosynthetically active radiation (PAR), (d) nitrate, (e) phosphate, (f) silicate, (g) turbidity, (h) chlorophyll-a, and (i) phytoplankton abundance.
Figure 2 Long description
The multiple graphs are arranged in a 3 by 3 layout and labeled (a) through (i). (a) The y-axis label is Temperature (degree celsius). The y-axis ranges from 30 to 33. The x-axis categories are Late-May, Mid-Aug, Late-Dec. Bar values: Late-May 32, Mid-Aug 33, Late-Dec 32. (b) The y-axis label is Salinity. The y-axis ranges from 20 to 36. The x-axis categories are Late-May, Mid-Aug, Late-Dec. Bar values: Late-May 35, Mid-Aug 27, Late-Dec 22. (c) The y-axis label is PAR (Einstein m superscript -2 day). The y-axis ranges from 35 to 55. The x-axis categories are Late-May, Mid-Aug, Late-Dec. Bar values: Late-May 52, Mid-Aug 51, Late-Dec 40. (d) The y-axis label is Nitrate (microM). The y-axis ranges from 0 to 14. The x-axis categories are Late-May, Mid-Aug, Late-Dec. Bar values: Late-May 5, Mid-Aug 13, Late-Dec 3. (e) The y-axis label is Phosphate (microM). The y-axis ranges from 0 to 2.0. The x-axis categories are Late-May, Mid-Aug, Late-Dec. Bar values: Late-May 1.4, Mid-Aug 1.9, Late-Dec 0.7. (f) The y-axis label is Silicate (microM). The y-axis ranges from 0 to 36. The x-axis categories are Late-May, Mid-Aug, Late-Dec. Bar values: Late-May 12, Mid-Aug 33, Late-Dec 16. (g) The y-axis label is Turbidity (NTU). The y-axis ranges from 0 to 24. The x-axis categories are Late-May, Mid-Aug, Late-Dec. Bar values: Late-May 10, Mid-Aug 24, Late-Dec 5. (h) The y-axis label is Chl-a (microg per L). The y-axis ranges from 0 to 40. The x-axis categories are Late-May, Mid-Aug, Late-Dec. Bar values: Late-May 22, Mid-Aug 40, Late-Dec 12. (i) The y-axis label is Abundance (times 10 to the power of 4 cells L superscript -1). The y-axis ranges from 0 to 50. The x-axis categories are Late-May, Mid-Aug, Late-Dec. Bar values: Late-May 10, Mid-Aug 45, Late-Dec 35.
Thalassiosira bloom during early SWM
During late May (onset of the SWM), a bloom of the centric diatom Thalassiosira spp. was observed in the intertidal zone, with an abundance of 93,000 cells L−1 and elevated chlorophyll-a concentration (22 µg L−1) (Figure 2). This bloom developed under conditions of high salinity (34.8 PSU), high light availability (PAR: 52.5 mol photons m−2 d−1), low turbidity, and moderate nutrient concentrations (nitrate: 4.5 µM; phosphate: 1.4 µM; silicate: 12.3 µM). Such environmental conditions are favourable for the growth of Thalassiosira spp. (Miyahara et al., Reference Miyahara, Nagai and Itakura1996), resulting in a green-dominant intertidal discolouration (Figure 3). Previous studies have shown that Thalassiosira can form resting spores that persist in sediments and germinate when favourable conditions arise, particularly during monsoon and post-monsoon periods characterized by nutrient enrichment (especially nitrate and silicate input from land runoff), reduced salinity, sufficient light penetration in sandy sediments, and hydrodynamic resuspension that reintroduces resting cells into the water column (Mitbavkar and Anil, Reference Mitbavkar and Anil2002). These resting stages may be one of the probable reasons contributing to the rapid initiation of blooms during the onset of the monsoon.
Intertidal diatom bloom observed during late May, resulting in (a–b) green-dominant discolouration (dark khaki) at Dona Paula intertidal zone and (c–d) microscopic views of the blooming diatom Thalassiosira spp.
Phytobenthic bloom of Navicula during late SWM
During mid-August (late SWM), a phytobenthic bloom dominated by the pennate diatom Navicula spp. was observed in the intertidal zone of Caranzalem. The bloom reached a high abundance of 5.41 × 10⁶ cells L−1 and formed dense accumulations in both the overlying intertidal waters and surface sediments, producing a distinct yellow-brown discolouration (peru) (Figure 4). This event coincided with peak nutrient concentrations (nitrate: 13 µM; phosphate: 2 µM; silicate: 32.2 µM), elevated turbidity (24 NTU), and reduced salinity (26.7 PSU; Figure 2), indicating strong freshwater influence. Navicula spp. are well adapted to intertidal environments and are known to persist year-round in sediments (Mitbavkar and Anil, Reference Mitbavkar and Anil2002). Sediment resuspension driven by waves and tides may enhance the redistribution of cells into the overlying intertidal waters, increasing their exposure to light and nutrients. Additionally, the presence of resting stages in deeper sediment layers may support rapid bloom development when environmental conditions become favourable (Mitbavkar and Anil, Reference Mitbavkar and Anil2002). Increased light availability during monsoon breaks may further facilitate bloom intensification (Barranguet et al., Reference Barranguet, Kromkamp and Peene1998).
Intertidal diatom bloom observed in mid-August, resulting in (a–b) yellow-brown discolouration (peru) of the Caranzalem intertidal zone and (c–d) microscopic views of the bloom-causing diatom Navicula spp.
Euglena bloom during NEM
During late December (peak NEM), an intertidal bloom dominated by Euglena spp. was observed, reaching an abundance of 3.29 × 105 cells L−1 and producing a green-dominant discolouration (dark khaki) of the intertidal zone (Figure 5). The bloom occurred under moderate environmental conditions, including PAR of 39.8 mol photons m−2 d−1, moderate turbidity (15 NTU), and relatively low nutrient concentrations (nitrate: 2.4 µM; phosphate: 0.7 µM). Microscopic observations showed cells containing numerous green intracellular bodies consistent with chloroplasts. Euglena spp. are known to exhibit mixotrophic growth and are commonly associated with organically enriched environments (Gibbs, Reference Gibbs1978). The occurrence of this bloom during a period of low rainfall may reflect localized organically enriched conditions potentially influenced by nearby freshwater and sewage inputs, although these inputs were not quantitatively assessed in the present study. Such conditions, combined with warm temperatures and relatively stagnant water, are favourable for Euglena proliferation. Similar observations have been reported in organically enriched and eutrophic environments (Munawar, Reference Munawar1972; Ratha et al., Reference Ratha, Jena and Adhikary2006; Li et al., Reference Li, Chen and Wang2017). Therefore, the bloom is interpreted as a response to localized organic matter enrichment, while the observed association among the organisms may indicate a potential ecological or symbiotic interaction; however, further molecular and ultrastructural studies are required to confirm this relationship.
Intertidal algal bloom observed in late December, causing (a–b) green-dominant discolouration (dark khaki) of the Caranzalem intertidal zone and (c–d) microscopic views of the bloom-causing Euglenophyte, Euglena spp.
Organic matter accumulation and brown-black discolouration
During the late SWM, a thick accumulation forming a dark brown-black patch (dark slate grey) was observed in the southern section of Caranzalem shore (Figure 6), resulting in pronounced intertidal discolouration. Microscopic examination identified this material as semi-degraded plant litter, likely derived from mangrove vegetation and terrestrial sources. This organic matter was transported into the estuary through monsoonal runoff and subsequently deposited in the intertidal zone through tidal and wave action. Previous studies have shown that increased precipitation enhances the export of organic carbon from mangrove ecosystems to adjacent coastal waters (Adame and Lovelock, Reference Adame and Lovelock2011). The decomposition of such material supports microbial and fungal communities and contributes to nutrient recycling within the ecosystem (Alongi, Reference Alongi2009; Mamidala et al., Reference Mamidala, Ganguly and Purvaja2023).
Intertidal discolouration observed during the late southwest monsoon in the southern section of Caranzalem intertidal zone, showing (a–b) dark brown-black patch (dark slate grey) and (c–d) microscopic views of the causative particles.
Image-based colour analysis
Image-based colour analysis revealed clear spatial variability in intertidal surface characteristics across the study sites. The derived RGB and HSV values showed distinct colour signatures associated with different discolouration events (Figure 7). Green-dominant (dark khaki) colouration associated with Thalassiosira and Euglena blooms, and yellow-brown (peru) discolouration associated with dense mats of Navicula spp., likely reflect differences in algal pigment composition, particularly chlorophyll- and fucoxanthin-rich diatom assemblages (Round et al., Reference Round, Crawford and Mann1990; MacIntyre et al., Reference MacIntyre, Geider and Miller1996). Diatoms are commonly referred to as ‘yellow or golden-brown algae’ due to the dominance of accessory pigments such as fucoxanthin, which impart characteristic golden-brown colouration to benthic mats and surface sediments (Kumar et al., Reference Kumar, Baweja, Sahoo, Sahoo and Seckbach2015). Such brownish-green surface discolouration is commonly associated with microphytobenthic assemblages inhabiting illuminated surface sediments (MacIntyre et al., Reference MacIntyre, Geider and Miller1996). In contrast, the organic-rich patch exhibited a dark brown-black appearance (dark slate grey) with comparatively lower brightness values. Unaffected intertidal areas exhibited pale cream colouration (dark grey) compared to bloom-affected regions.
Colour characterization of the intertidal zone using image analysis shows the (a) ROI extracted from the original photograph, (b) average RGB colour, and (c) pure hue representation of the ROI. Panels a3–c6 represent the discolouration features shown in Figures 3–6, respectively, while the bottom row (a7–c7) shows the natural colour of the intertidal zone. (d) Workflow of the image-based colour analysis used for intertidal discolouration characterization.
Figure 7 Long description
The image consists of two main sections: intertidal zone photos with color data and a flowchart. The intertidal zone section includes: - (a3) Intertidal zone photo labeled ′ROI′ with averaged color RGB (175, 185, 123) and hue representation #ffb600. - (a4) Photo with RGB (191, 153, 59) and hue #ff9600. - (a5) Photo with RGB (185, 167, 116) and hue #ffb600. - (a6) Photo with RGB (53, 44, 39) and hue #ff6000. - (a7) Photo with RGB (178, 178, 155) and hue #ffb600. The flowchart section includes steps: - Load Image - Select ROI - Extract RGB - Convert to HSV - Classify hue - Create colour panels - Save output image.
Standardized CSS3/Web colour classifications derived from RGB values further distinguished the discolouration patterns and reduced subjective interpretation of colour descriptions. The generated composite images, including ROI-based RGB and HSV visualizations with corresponding HEX values and standardized colour names (Figure 7b), demonstrated that phytobenthic blooms and organic matter accumulation produce distinct and quantifiable colour signatures that can be consistently characterized using digital image analysis techniques. Similar image-based environmental monitoring approaches have demonstrated that digital imagery can effectively capture ecologically meaningful optical variations and provide reliable proxies for environmental surface characteristics (Richardson et al., Reference Richardson, Jenkins and Braswell2007). Comparable image-based observations of benthic microphytobenthic biofilms have also been reported from the Guadalquivir Estuary, Spain, where diatom-dominated intertidal biofilms formed visually distinct bright brown surface patches detectable through UAV and multispectral imaging approaches (Román et al., Reference Román, Oiry and Davies2024). Their study demonstrated that colour and spectral characteristics of intertidal biofilms can be effectively linked with benthic microalgal biomass and pigment composition. The present observations further support the applicability of low-cost digital image analysis for documenting transient intertidal discolouration events and differentiating biologically and organically induced surface features. However, microscopic observations remained essential for confirming the dominant causative organisms associated with each discolouration event.
Ecological implications of intertidal discolouration
The observed discolouration events were associated with increased biological productivity in the intertidal zone. Phytobenthic blooms and organic matter accumulation may enhance food availability for microbial communities, meiofauna, and macrobenthic organisms, thereby contributing to benthic productivity and trophic transfer within intertidal ecosystems (MacIntyre et al., Reference MacIntyre, Geider and Miller1996). However, such events may also have ecological consequences. Blooms of opportunistic taxa such as Euglena spp. may indicate organic enrichment and declining water quality, particularly in areas influenced by sewage inputs. Similarly, dense algal mats and organic deposits can alter sediment oxygen dynamics and affect benthic community structure (Alongi, Reference Alongi2009). Comparable intertidal microphytobenthic blooms and discolouration events have been reported from several tropical and subtropical estuarine systems, particularly under nutrient-enriched and shallow-water conditions (Mandal et al., Reference Mandal, Dutta, Das and Mukherjee2021). Environmental factors such as salinity, temperature, light availability, tides, and nutrient enrichment are known to regulate benthic microalgal biomass and biofilm development in tropical intertidal habitats. In contrast, temperate estuarine and mudflat systems have been more extensively investigated, where microphytobenthic blooms are recognized for their ecological roles in sediment stabilization, nutrient cycling, benthic productivity, and trophic interactions (Cahoon, Reference Cahoon1999; Underwood and Paterson, Reference Underwood and Paterson2003; Hope et al., Reference Hope, Paterson and Thrush2020). Similar to observations from temperate systems, the present study suggests that episodic phytobenthic blooms in tropical estuarine intertidal zones may also represent ecologically important but comparatively underreported phenomena. Although these events may temporarily enhance ecosystem productivity, prolonged or intensified occurrences could disrupt trophic balance and ecosystem functioning. Continued monitoring is therefore essential to understand their long-term ecological impacts under changing environmental and anthropogenic conditions.
Limitations and future research perspectives
The present study was primarily based on short-term field observations, and although discolouration events and their associated dominant organisms were monitored during bloom occurrence, hydrographic parameters were not continuously recorded throughout the complete pre-bloom and post-bloom phases. In addition, bloom initiation processes, including possible development from resting spores or dormant stages, were not investigated. Comparative analyses between bloom and non-bloom conditions were constrained due to the limited availability of corresponding hydrographic datasets during non-bloom periods. Based on the present monitoring observations, these discolouration events appeared sporadic and short-lived within the study period; however, long-term observations are needed to determine their recurrence patterns and interannual variability. The observations were conducted during 2023–2024, a period influenced globally by El Niño conditions (Peng et al., Reference Peng, Xie and Miyamoto2025) and regional climatic variability. Although the present study did not specifically evaluate climate anomalies or long-term rainfall deviations, such large-scale climatic factors may influence hydrographic conditions, nutrient dynamics, and the frequency of intertidal discolouration events. Integrated climatic and hydrological investigations are therefore required to better assess potential climate-related influences on these events. Variations in illumination, cloud cover, sediment wetness, and tidal conditions should be carefully considered, as they may influence RGB- and HSV-derived colour measurements obtained under natural field conditions. These limitations restrict detailed interpretation of bloom dynamics, environmental drivers, ecological interactions, and life-cycle progression associated with the observed intertidal discolouration events.
Benthic biodiversity associated with the affected intertidal habitats was not comprehensively assessed. However, opportunistic field observations, complemented by microscopic examinations, indicated that the exposed intertidal zones supported diverse faunal assemblages, including foraminifera, ostracods, gastropods, and polychaete worms (Figure 8a–d). During low tide, the exposed sediment surface also attracted numerous shorebirds such as seagulls, crows, and black kites actively foraging within the intertidal habitat (Figure 8e–i). Bird congregation and associated faecal deposition were frequently observed in the affected zones (Figure 8e–g) and may contribute localized nutrient enrichment capable of supporting algal growth and organic matter accumulation. However, these observations were qualitative in nature and were not supported by dedicated ecological sampling or nutrient-budget analyses. Therefore, the ecological interactions and trophic influences associated with these intertidal discolouration events require further targeted investigation.
Faunal diversity and foraging activity in the exposed intertidal zone of Caranzalem during various low tide events. (a) Foraminifera under the microscope, (b) ostracods under the microscope, (c) gastropods, and (d) polychaete worm. Panels (e) through (i) depict the exposed intertidal area at low tide, revealing numerous shorebirds including seagulls, crows, and black kites actively foraging for invertebrates. The receding water exposes a rich feeding ground, attracting a variety of avian predators. In turn, these birds release faecal matter into the habitat (g), contributing nutrients that promote algal growth.
Conclusion
This study documents four distinct phytobenthic bloom events in the intertidal waters of an estuary in Goa, India, highlighting their seasonal variability, causative organisms, and environmental drivers. While plankton blooms in offshore waters are well studied, intertidal blooms remain underreported. Our findings show that blooms of Thalassiosira, Navicula, and Euglena spp. correspond to seasonal shifts in salinity, nutrient availability, light, and turbidity. Notably, a bloom dominated by Euglena spp. during the NEM indicates the proliferation of mixotrophic taxa under organically enriched conditions. Additionally, the deposition of mangrove litter during the monsoon may contribute to discolouration and support microbial and fungal communities. However, these ecological interactions were not investigated in the present study and require further examination in future work. These blooms may enrich intertidal zones, fostering diverse microbial, meiofaunal, and macrobenthic communities and supporting higher trophic levels. This study highlights the potential ecological significance of phytobenthic blooms and detrital inputs in intertidal zones, while emphasizing the need for continued monitoring and future ecological investigations to better understand their impacts on intertidal productivity, habitat conditions, and food web dynamics under changing environmental and anthropogenic pressures.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/S0025315426101416.
Acknowledgements
The authors express their gratitude to the Director, CSIR-National Institute of Oceanography, India, for providing the necessary facilities and encouragement. They sincerely thank Dr Sachitra Kumar Ratha of the Algology Laboratory, CSIR-NBRI, for his valuable assistance in the identification of Euglena sp. They are also thankful to the panel members of the World Ocean Science Congress Conference 2024, held in Chennai, for their valuable feedback and constructive comments on the oral presentation of these results under the sub-theme Marine Biodiversity and Ocean Ecosystem (Abstract ID: WOSC/2024/ABS/147), which considerably contributed to the improvement of this study. They acknowledge AI tools (ChatGPT) for grammatical corrections. The contribution number is 7670.
Author contributions
K.C. conceived the study, designed the methodology, participated in field observations and sample collection, analysed the data, prepared figures, and wrote the manuscript. S.K.R., B.K.S., A.S., and H.M.K. participated in field observations, sample collection, and data acquisition. All authors contributed to manuscript revision and approved the final version.
Declaration of interest
The authors declare no conflict of interest.
Data availability statement
The field-measured data that support the findings of this study are available from the corresponding author upon reasonable request.
Ethics and approach
Not applicable.
