Study area

An experimental seaweed culture site was designed in the sheltered intertidal zones of Chowfoldondi (91°59′38.8′′ E and 21°30′11.7′′ N) of Cox’s Bazar district (Fig. 2), along the Bay of Bengal’s north-eastern coast. The study area was on the Bakkhali River (the second largest river in the district), which has moderate wave action and is directly connected to the Bay of Bengal. The average water depth of the culture site was 700 cm. This area is considered a biologically diverse ecosystem, having intertidal mudflats with various salt marshes, seagrasses, cord grasses, seaweeds and also some mangrove vegetation. Additionally, this place is a safe habitat for numerous types of fish, reptiles, oysters, mussels, crabs, snails, shrimp and so on. The yearly average rainfall and temperature were 3770 mm and 25.6 °C, respectively, in the Cox’s Bazar district. This tropical climatic area’s wind speed average was about 8.3 miles per hour. The study was performed for a period of 90 days from January 2022 to March 2022.

Figure 2
figure 2

Location of the G. longissima culture area at Chowfoldondi, Cox’s Bazar (the map was generated by using QGIS version 3.24.1.; retrieved from https://www.filehorse.com/download-qgis/old-versions/).

Seed collection

Young, wild G. longissima seeds were initially collected from the intertidal zones of the Nuniachara coast (91°57′52.0′′ E and 21°28′28.9′′ N) of Cox’s Bazar Sadar. This sand flat site is a natural bed of seaweed with some other seagrass, salt marsh and mangrove vegetation. Permission to collect seaweed samples and their culture practices was obtained from the local government in accordance with local and national legislation. The botanical identification of seaweed species was checked and confirmed through the published literature38,39. Dr. Md. Enamul Hoq, Former Director of BFRI, validated the botanical identification of the seaweed species, as the voucher specimen had already been placed at BFRI herbarium [BFRI (MFTS-RS-18/19-038). A fresh sample was collected in an open box with adequate seawater and an aeration facility and then immediately transferred to the cultivation site to maintain fresh quality.

Experimental culture raft setup

Several bamboo poles (7.0–10.0 cm diameter) made into square (5 m × 5 m) frames were prepared to provide the seaweed culture layout. The four corners of the raft were steadied tightly between and among themselves to keep the raft shape intact. Two more bamboos were fastened tightly at the opposite ends to make the structure stronger. Four recycled plastic drams were attached to the structure’s four corners, ensuring that it was always floating on the water. A 1.50 cm mesh size plastic net was placed in the lower part of the frame to minimise the wave action, and crop loss caused by plant rupture from the base, especially during adverse weather. All of the rafts were rope-tied, placed in the culture site and anchored to help stabilise the structure. The anchor of the structure was placed in such a way that it could raise and fall vertically during the tidal action. Each experiment had three replications (Fig. 3).

Figure 3
figure 3

(A) Floating raft structure for G. longissima culture at Chowfoldondi and (B) G. longissima grown in floating raft.

Seaweed seeding

The younger pieces of G. longissima were used for seeding with an average of 5 ± 0.4 g of fresh weight in each knot and 5 cm size in the rope twists. No fertiliser, growth hormone or any other chemicals were used during the culture period. Partial harvesting was done when the seaweed reached an average standard length. The partial harvesting took place by cutting off the algae hanging on the surface, allowing the base on the surface to expand further. Seaweed biomass yield production was measured as the fresh weight of seaweed per unit culture area (kg/m2) and was calculated using Eq. (1) 40.

$${text{Y }} = , left( {{text{W}}_{{text{n}}} – {text{ W}}_{0} } right)/{text{A}}$$

(1)

Here, Y is the seaweed biomass yield production; Wn is the raw weight on day n; W0 is the beginning raw weight; A is the culture unit’s area.

Daily growth rate % was calculated using Eq. (2) 41.

$${text{DGR }}% , = {text{ ln }}left( {{text{W}}_{{text{f}}} /{text{ W}}_{{text{o}}} } right) , /{text{ t }} times { 1}00$$

(2)

Here, Wf is the final raw weight (g) at t day; Wo is the initial raw weight (g); t is the cultivation period (days).

Water quality variables

The culture site was studied throughout the 90-day culture period from January 2022 to March 2022. Sampling was carried out twice a month. Multiple water quality parameters such as temperature, pH, dissolved oxygen (DO), salinity, TDS (total dissolved solids), transparency, alkalinity, ammonia, nitrite, nitrate, phosphate and silica were measured. Temperature, pH, dissolved oxygen, salinity and TDS were all measured on the spot using a HANNA HI-98194 multiparameter. Measurement of water transparency was performed with a Secchi disc. Water samples from 0 to 100 cm depth was collected (Van Dorn water sampler) and immediately transported to the laboratory. Alkalinity, ammonia, nitrite, nitrate, phosphate and silica in water were analysed following the methods of HANNA (Hanna COD and Multiparameter Bench Photometer, 230 V-HI83099 procedure manual).

Effect of rope material

This experiment was carried out to explore the rope material’s effect on the biomass yield performance of G. longissima. In this study, four types of rope materials (T1 = nylon rope, T2 = plastic rope, T3 = coir rope: acquired from coconut husk, and T4 = jute rope) were used. Among them, coir and jute ropes were biodegradable, while the other two were non-biodegradable. The best biomass yield performances along with the rope materials’ sustainability in saline water were observed.

Effect of culture type

In this study, different culture types, such as long line and square net methods, were used to observe the biomass yield performance of G. longissima. Here, T1 stands for the long line method and T2 stands for the square net method.

Effect of raft shape

In this experiment, different shaped rafts were prepared and observed for their effect on the biomass yield performance of G. longissima. Here, T1 (square shape) and T2 (triangular shape) rafts were prepared and observed for their biomass yield performance.

Effect of seeding intensity

This experiment was conducted to observe the effect of seeding intensity on the biomass yield production of G. longissima. Here, T1, T2 and T3 indicate 50, 100 and 150 seeds per square meter, respectively. This results in 250 g, 500 g and 750 g initial seaweed seeds per square meter for cultivation. During harvesting, fresh seaweed samples were collected separately and washed carefully with running fresh water to remove dirt and any other impurities. The fresh sample was then weighted with the help of a digital balance.

Effect of harvesting phase

In this study, after the initial seeding, harvesting was performed at four different intervals (T1 = 15 days, T2 = 30 days, T3 = 45 days and T4 = 90 days). Here, in the case of T1, a total of six partial harvests; in the case of T2, a total of three partial harvests; in the case of T3, a total of two partial harvests; and in the case of T4, zero partial harvests were performed throughout the 90 day of the culture period. The best harvesting intervals were evaluated through the maximum biomass yield performance of G. longissima.

Effect of water depth

In this study, several vertical culture units were placed at different water levels in the open sea. T1 represents the first unit that was placed at the water’s surface, T2 represents the second unit that was placed below the first one at a 25 cm depth from the water’s surface, and T3 represents the third unit that was placed below the second one at a 50 cm depth from the water’s surface. The possibility of vertical expansion of seaweed culture was evaluated through the maximum biomass yield performance of G. longissima at each depth.

Seasonal appearance of epiphytic algae

Epiphytic algal occurrences on G. longissima and the associated culture materials were documented during the 90-day (January–March 2022) cultivation period. Seaweed samples were collected, washed with clean water and preserved in silica gel in sample vials. Finally, seaweed samples were confirmed through microscopic examination and cross-checked against the published literature38,39.

Cost–benefit analysis of seaweed culture

The cost–benefit analyses of G. longissima cultures for 20 rafts (5 m × 5 m) in a 180-day culture period was examined. The economic turnover of G. longissima cultures through biomass yield production can be estimated. Investment includes all input materials and maintenance costs. Based on the local market, all prices of investments were included and expressed in US$.

Statistical analysis

The experimental data was analysed using standard statistical techniques. Statistical package SPSS version 20.0 (IBM Co., Chicago, IL) was used to examine the data. One way ANOVA and T-test were used to determine the significance of each parameter among different treatments. The level of significance was set at a 95% probability level.

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