Tuft Dynamics and the Reproductive Phenology of Zostera caespitosa on the Southern Coast of Korea

Abstract

The aim of study is to determine which environmental factors could influence the biological traits of Z. caespitosa, a unique tuft-forming seagrass. This study examined the dynamics of tufts and the growth of Z. caespitosa, along with the environmental factors. The reproductive traits were also examined to estimate the potential importance of sexual reproduction in population persistence. The density of tufts remained constant, and no new tufts produced through seedling recruitment were observed throughout the sampling period. On the other hand, the tuft size and growth exhibited clear seasonal manners and strong correlations with the water temperature, indicating that water temperature regulates the tuft dynamics and growth. The optimal growth temperature for Z. caespitosa at the study site was approximately ~22.5 °C during early summer, with growth severely inhibited during periods of high-water temperatures. Z. caespitosa was characterized by a low flowering percentage and fewer inflorescences, resulting in extremely low potential seed production. Z. caespitosa maintained its populations through clonal tuft growth with low sexual reproduction and restricted growth at high water temperatures. Hence, this seagrass species may be vulnerable to disturbances, exhibiting low resilience and facing a high risk of becoming a threatened species in coastal waters.

1. Introduction

Population dynamics and phenology are fundamental areas of plant ecology. These studies are typically conducted to understand population changes, phenology, and the influences of biotic and abiotic factors, providing essential information for more complex communities [1,2]. Seagrasses are angiosperms that have adapted to living in seawater [3]. Among the seven seagrass bioregions, the northwestern Pacific is notable for its relatively high diversity of seagrass species [4]. Three Zostera species (Z. asiatica, Z. caulescens, and Z. caespitosa) and two Phyllospadix species (P. japonicus and P. iwatensis) are endemic to the northwestern Pacific region [4]. Despite the extensive research on cosmopolitan seagrass species, including Z. marina and Posidonia oceanica, information on the population dynamics and phenology of endemic species in the northwestern Pacific is relatively scarce because of their limited distributional range [5,6].

In terrestrial ecosystems, caespitose grasses are distributed across all continents, from the Arctic to the Subantarctic zones [7]. They represent unique growth characterized by the dense clustering of ramets within individual clones and the lack of rhizomes, resulting in the development of tufts or dense clusters of shoots, like rice [8]. Although tuft-forming plants are rare in marine environments, among seagrasses, Z. caespitosa exhibits a tuft-forming growth pattern in the soft bottoms of coastal ecosystems [9]. Z. caespitosa is endemic to northwestern Pacific coasts and is found only in Korea, Northeast China, and northern Japan [4]. Compared to other species in the genus Zostera, which have rhizomes that grow horizontally in sediment, Z. caespitosa has sub-erect rhizomes with extremely short internodes and persistent sheaths, forming a tufted appearance consisting of densely packed individual shoots [10,11]. Z. caespitosa does not produce creeping rhizomes and grows in isolated tufts, clearly distinguishing it from other Zostera species.

The biological characteristics of seagrasses, such as productivity, biomass, and shoot density, are regulated mainly by the irradiance, water temperature, and ambient nutrient concentrations [12]. Because seagrasses require a sufficient amount of underwater irradiance for photosynthesis, their survival and growth are significantly affected by the changes in underwater irradiance [12,13]. Chronic and sudden decreases in underwater irradiance often lead to the decline of seagrass meadows [14,15]. Water temperature is a pivotal parameter influencing the growth and reproductive phenology of the seagrasses [12,16,17,18,19]. In addition, seagrasses should assimilate large amounts of inorganic nutrients because of their high productivity. They can absorb nutrients from the sediment pore water and the seawater through leaves [20,21]. Nutrient concentrations in the seawater and sediment also can be critical regulatory factors for seagrass growth.

Only a few studies have been examined on the growth of Z. caespitosa because this species has a very limited distribution range [11,22,23]. To the authors’ knowledge, there is no research on the peculiar growth form of Z. caespitosa, specifically its tuft formation. The aim of this study is to identify which environmental parameters affect the seasonal growth and tufts of Z. caespitosa. We hypothesized that changes in the growth and tufts of Z. caespitosa are closely related to water temperature and underwater irradiance. This study examined the growth and dynamics of Z. caespitosa tufts with simultaneous measurements of environmental parameters, including light availability, water temperature, and ambient nutrients, to assess the growth dynamics. In addition, the percentage of reproductive shoots and potential seed production of Z. caespitosa were measured to estimate the potential roles of sexual reproduction in population persistence.

2. Materials and Methods

2.1. Study Site

The research was performed in Jangmok Bay (34°59′ N, 128°40′ E) (Figure 1). This bay is a semidiurnal system (an amplitude fluctuation: ~2.5 m) and consists of a mixture of sand and rubble, ranging between 54% and 71% [11]. Two Zostera species are distributed sequentially in shallow subtidal zones. Z. marina grows at a water depth of 1–2 m below the mean low water (MLW) line, whereas Z. caespitosa is found at a water depth of 1.5–4 m. The experiment was performed in a monospecific Z. caespitosa meadow at a water depth of ~3 m below the MLW line.

2.2. Environmental Parameters

The water temperature and underwater photon flux density (PFD) were measured continuously from February 2021 to February 2022. Underwater PFD and water temperature were recorded every 15 min using a HOBO pendant temperature/light logger (Onset Computer Corporation, Bourne, MA, USA). Prior to deployment, the sensor was pre-calibrated using simultaneous quantum measurements with the LI-193SA spherical quantum sensor and LI-1400 data logger (Li-Cor, Lincoln, NE, USA). To reduce fouling by epiphytes and sediment, the light sensor was routinely cleaned every 4 weeks. Daily irradiance was determined by summing of values throughout an entire day (24 h). The recorded water temperature was averaged on a daily basis.

Four replicates of surface seawater were sampled monthly from February 2021 to February 2022 to determine the concentrations of inorganic nutrients in the seawater. In addition, 4 replicates of sediment were sampled randomly in the monospecific meadow of Z. caespitosa to a sediment depth of ~13 cm using an acrylic corer during the study period. The samples were kept on ice in a cool box and then frozen until laboratory analysis could be conducted. The sediment samples were centrifuged (8000× g, 20 min) to obtain the pore water. The dissolved inorganic nitrogen and phosphate concentrations were measured by Parsons et al. [24].

2.3. Biological Measurements

Owing to the tuft-forming growth pattern of Z. caespitosa, it is not appropriate to monitor the changes in biomass and shoot density of this species using the traditional quadrate method, which is typically adopted for meadow-forming seagrass species [25,26]. Therefore, permanent quadrats (1 × 1 m; n = 4) were established in a monospecific Z. caespitosa meadow for mapping the location of tufts (Figure S1). In addition, the number of shoots in individual tufts was recorded to determine the tuft size. The tuft dynamics were then examined, and the shoot density per unit area (m⁻²) was measured. During the flowering season, the reproductive traits such as the reproductive shoot density, the number of inflorescences, the number of seeds, and potential seed production were assessed.

The leaf productivity was estimated using a modified plastochrone method [27,28]. Approximately 15 randomly chosen shoots near the permanent quadrats were marked by piercing the sheath with a <0.2 mm diameter fine hypodermic needle. After four weeks, the marked shoots were collected and washed with tap water. The leaf plastochrone interval was determined by dividing the marking duration (days) by the number of new leaves formed after marking. The dry weight of the third leaf, typically the youngest mature leaf, was measured at each sampling time. The leaf productivity per shoot was determined using the equation below:

Leaf productivity (mg DW shoot⁻¹ day⁻¹) = dry weight of a mature leaf (mg shoot⁻¹)/plastochrone interval (day)

The areal productivity (g DW m⁻² day⁻¹) was then calculated by multiplying the leaf productivity per shoot by the shoot density per unit area.

The collected shoots for productivity assessments were used to measure the morphological characteristics of Z. caespitosa, such as the shoot height, the length and width of sheath, the number of leaves, and the internode length of rhizomes. The collected shoots were completely cleaned to remove sediments and epiphytes, divided into above- and below-ground tissues, and then dried at 60 °C for individual shoot weight. The biomass of above- and below-ground tissues was determined by multiplying the individual weight of the respective tissues by the shoot density per unit area.

2.4. Statistics

SPSS 27.0 software (SPSS Inc., Chicago, IL, USA) was used for statistical analyses. One-way analysis of variance (ANOVA) was used to identify significant (p-values < 0.05) differences in biological and environmental variables across the sampling months. The results of the one-way ANOVA were evaluated using F-statistics, representing the F-value, degree of freedom, and p-value. If significant differences were found, the Student–Newman–Keuls test was used to identify the differences among the means. Before the one-way ANOVA, variables were checked for the parametric assumptions. Data were log-transformed when necessary (underwater irradiance, water temperature, shoot morphology, and leaf productivity). Correlation analysis was conducted to assess the relationships between environmental and biological variables. Regression analysis using a peak model was performed to identify the relationship between Z. caespitosa growth and the water temperature when the water temperature increased gradually. All graphs of the results were generated using SigmaPlot 14.0 software, respectively.

3. Results

3.1. Underwater PFD, Water Temperature, and Inorganic Nutrients

The daily average irradiance exhibited notable fluctuation, and the monthly average irradiance varied significantly (F_12,385 = 19.69, p < 0.001) across sampling periods (Figure 2A). There was a general trend in increasing PFD during winter and spring months, followed by a decrease in the summer and fall; it was lowest (6.5 mol photons m⁻² day⁻¹) in December 2021 and highest (24.9 mol photons m⁻² day⁻¹) in April 2022. The water temperature also showed significant (F_12,385 = 909.05, p < 0.001) seasonal variations, ranging from 6.9 °C in February 2022 to 27.8 °C in August 2021 (Figure 2B). The average water temperature was 17.4 °C at the study site from February 2021 to January 2022.

The concentrations of NH₄⁺, NO₃⁻+NO₂⁻, and PO₄³⁻ in the seawater varied significantly (F_12,47 = 211.04, F_12,51 = 41.93, and F_12,51 = 17.39, respectively, all p < 0.001) with the sampling months, but did not show a clear seasonal pattern (Figure 3). The seawater NH₄⁺ ranged from 0.1 to 4.5 μM, whereas the water-column NO₃⁻+NO₂⁻ concentrations ranged from 0.5 to 5.5 μM during the experiment period (Figure 3A,B). The NH₄⁺ and NO₃⁻+NO₂⁻ concentrations in the seawater were typically less than 0.5 μM and 1.5 μM, respectively, except during September–October and August–November. The average PO₄³⁻ concentration was 0.4 μM over the experimental period, ranging from 0.1 to 0.7 μM (Figure 3C).

The sediment pore water NH₄⁺ and NO₃⁻+NO₂⁻ fluctuated significantly (F_12,50 = 6.66, p < 0.001 and F_12,51 = 3.13, p < 0.01, respectively) with the sampling time (Figure 3D,E). The average NH₄⁺ and NO₃⁻+NO₂⁻ in sediment pore water during the sampling period were 83.5 μM and 2.9 μM, respectively. The PO₄³⁻ in the pore water was typically < 10 μM throughout the sampling period, ranging from 5.2 μM to 31.4 μM with an average of 9.7 μM, except from June to August (Figure 3F).

3.2. Tuft Dynamics and Reproductive Characteristics of Z. caespitosa

Figure 4 shows the monthly changes in the size distribution of the tuft. In the initial sampling month (February 2021), the most abundant tuft size consisted of 10–20 shoots, with the maximum tuft size being 40–50 shoots tuft⁻¹. The tuft size increased until August, and tufts with more than 100 shoots were observed in July and August 2021. From August 2021 to November 2021, the tuft size decreased, and tufts with 10–20 shoots were the most abundant, with the maximum tuft size being 30–40 shoots in November 2021. The mean tuft size showed significant (F_12,51 = 10.68, p < 0.001) variations, ranging from 16.0 shoots tuft⁻¹ in November 2021 to 52.6 shoots tuft⁻¹ in July 2021. On the other hand, the tuft density of Z. caespitosa was not significantly (F_12,51 = 0.22, p = 0.997) different over the sampling period. It remained constant, between 10 and 12.5 tufts m⁻², throughout the experimental period (Figure 5B).

The vegetative shoot density of Z. caespitosa exhibited significant (F_12,51 = 18.73, p < 0.001) seasonal variations, ranging from 172.3 shoots m⁻² in November 2021 to 533 shoots m⁻² in July 2021 (Figure 5C). The reproductive shoots of Z. caespitosa were first observed in February 2021 and persisted until June at the study site (Figure 5C;

Table 1. Proportion of reproductive shoots to total shoot density (% reproductive shoots), reproductive shoot height, number of inflorescences per shoot, and number of seeds per inflorescence of Zostera caespitosa at the study site during flowering period.

Variables	February	March	April	May	June
% of reproductive shoots	0.4 ± 0.3	4.9 ± 0.8	5.8 ± 1.3	4.9 ± 0.7	0.1 ± 0.1
Reproductive shoot height (cm)	78.9 ± 5.2	77.2 ± 5.9	135.6 ± 6.7	129.5 ± 6.3	145.7 ± 6.7
Number of inflorescences per shoot	3.5 ± 0.3	2.8 ± 0.5	5.4 ± 1.2	7.1 ± 0.9	5.8 ± 1.3
Number of seeds per inflorescence				10.9 ± 0.3	5.0 ± 0.6

). The reproductive shoot density varied significantly (F_4,19 = 34.41, p < 0.001) during the flowering season, ranging between 0.8 and 24 shoots m⁻² (Figure 5C). The proportion of reproductive shoots to total shoot density (% reproductive shoots) was lowest (0.1%) in June 2021 and highest (5.8%) in April 2021 during the flowering season (Table 1). The number of inflorescences per shoot ranged from 2.8 in March 2021 to 7.1 in May 2021. Mature seeds were observed in May and June of 2021, with a maximum of 10.9 seeds per inflorescence (Table 1). The potential seed production of Z. caespitosa was 1857 seeds m⁻² at the study site.

3.3. Shoot Morphology, Biomass, and Production

The shoot height and sheath length of Z. caespitosa exhibited significant seasonal variations (F_12,141 = 29.51, p < 0.001 and F_12,141 = 43.36, p < 0.001, respectively) (Figure 6A,B). The shoot height ranged from 55.7 cm in November 2021 to 118.2 cm in July 2021, whereas the sheath length ranged from 9.2 cm in February 2022 to 21.1 cm in July 2021. The sheath width also varied significantly with the sampling time (F_12,147 = 28.70, p < 0.001) remaining constant from February to September and decreasing sharply in the fall (Figure 6C). The quantity of leaves per shoot varied significantly across the different sampling periods (F_12,147 = 6.77, p < 0.001) but did not show clear seasonal variations; it ranged between 3.5 and 5.5 leaves throughout the experimental period (Figure 6D). The rhizome internode length varied significantly (F_12,566 = 2.49, p < 0.01) among the sampling times (Figure 6E). The rhizome internode length was less than 1.7 mm throughout the experimental period. The individual shoot weights of the above- and below-ground tissues differed significantly (F_12,136 = 5.21, p < 0.001 and F_12,147 = 18.42, p < 0.001) across the sampling periods. On the other hand, they exhibited different seasonal patterns between the two types of tissues (Figure 6F).

The total and above-ground biomass of Z. caespitosa showed significant seasonal variations (F_12,51 = 67.40, p < 0.001 and F_12,51 = 76.31, p < 0.001, respectively), with increases in spring and early summer, followed by decreases in fall and winter (Figure 7A,B). The ranges of the total and above-ground biomass were between 104.7 and 861.5 g DW m⁻² and between 101.9 and 846.0 g DW m⁻², respectively. The below-ground biomass varied significantly with the sampling time (F_12,51 = 64.44, p < 0.001) but did not display a distinct seasonal pattern. The lowest value (2.8 g DW m⁻²) was recorded in November 2021, while the highest value (31.6 g DW m⁻²) was observed in April 2021 (Figure 7B). The ratio of above- to below-ground biomass was lowest (5.9) in March 2021 and highest (95.0) in September 2021 (Figure 7C).

The leaf productivity per shoot and unit area exhibited significant seasonal variations (F_12,141 = 15.40, p < 0.001 and F_12,51 = 90.99, p < 0.001, respectively) (Figure 8A,B). The productivity increased in spring and early summer, decreased rapidly in late summer and fall, and reached its minimum in winter. The leaf and the areal leaf productivity were lowest (4.3 mg DW shoot⁻¹ day⁻¹ and 1.2 g DW m⁻² day⁻¹, respectively) in February 2022 and highest (22.1 mg DW shoot⁻¹ day⁻¹ and 10.6 g DW m⁻² day⁻¹, respectively) in June 2021 (Figure 8A,B).

3.4. Relationship between Growths and Environmental Factors

The areal leaf productivity and total biomass of Z. caespitosa were not significantly correlated with the underwater irradiance, while the shoot density correlated with the underwater irradiance (p < 0.05) (

Table 2. The Pearson correlation coefficients between seagrass parameters and environmental factors. Coefficients in bold represent a statistically significant correlation.

	Underwater Irradiance	Water Temperature	Water Column			Sediment Pore Water
			NH4+	NO3−+NO2−	PO43−	NH4+	NO3−+NO2−	PO43−
Areal leaf productivity	0.368	0.738 **	−0.064	−0.140	0.291	0.077	0.555 *	0.651 *
Total biomass	0.418	0.586 *	−0.088	−0.177	0.221	0.104	0.546	0.623 *
Shoot density	0.569 *	0.706 **	−0.153	−0.281	0.054	0.069	0.432	0.582 *

* and ** indicates significant level of p < 0.05 and p < 0.01, respectively.

). The areal leaf productivity, total biomass, and shoot density were significantly correlated to the water temperature (p < 0.01, p < 0.05, and p < 0.01, respectively), but were not associated with any nutrient concentrations in the seawater (Table 2). Among the dissolved inorganic nitrogen in sediment pore water, only the NO₃⁻+NO₂⁻ showed a significant relationship with the areal leaf productivity. All biological characteristics of Z. caespitosa were significantly correlated with the PO₄³⁻ concentrations in the sediment pore water (Table 2).

The areal leaf productivity, total biomass, and shoot density increased rapidly during spring, reaching their peak in early summer (June or July). They then decreased dramatically when the water temperature exceeded ~22.5 °C, according to the equations of the peak models below (Figure 9). After reaching the optimal growth temperature in June or July, areal leaf productivity, total biomass, and shoot density decreased continuously regardless of the water temperature (Figure 8). Regression analysis using a peak model between Z. caespitosa growth and water temperature revealed significant relationships when the water temperature increased gradually (February to August) (Figure 9). The equations of a peak model for areal leaf productivity, total biomass, and shoot density in relation to water temperature (Equations (1), (2) and (3), respectively) are as follows:

$$y=10.8263\times e^{-0.5{\left({\displaystyle \frac{x-22.2571}{5.2299}}\right)}^2}$$

(1)

$$y=877.6350\times e^{-0.5{\left({\displaystyle \frac{x-22.5564}{7.3973}}\right)}^2}$$

(2)

$$y=508.0621\times e^{-0.5{\left({\displaystyle \frac{x-22.7557}{12.6478}}\right)}^2}$$

(3)

4. Discussion

4.1. Tuft Dynamics and Sexual Reproduction of Zostera caespitosa

Compared to meadow-forming species in the genus Zostera, Z. caespitosa forms a tufted appearance of dense shoots due to sub-erected rhizomes with extremely short internodes. Hence, examining the changes in the tufts is more appropriate for understanding the growth dynamics of this species than focusing on the variations in individual shoots. In this study, the number of tufts per unit area (tuft density) remained relatively constant throughout the experimental period, ranging from 10 to 12.5 m⁻². Although new tufts of Z. caespitosa could be produced during winter and early spring through seedling recruitment via seed germination, followed by an increase in size during clonal growth, no new tuft formation via sexual reproduction was observed at the study site. Instead, new tufts were formed during the fall through the fragmentation of large tufts (Figure S1). Throughout the sampling period, the tuft density decreased slightly as some tufts disappeared. Furthermore, as the size of two nearby tufts increased through clonal growth, the two tufts merged into a single tuft at the study site (Figure S1). Z. caespitosa tufts cannot spread laterally because the species does not have creeping rhizomes. Thus, the formation and disappearance of tufts were rarely observed under natural environmental conditions (Figure S1). On the other hand, the tuft size (number of shoots per tuft) showed clear seasonal variation, with increases in spring and summer, followed by decreases in fall and winter. Tufts with fewer than 20 shoots were the most abundant during winter, and the tuft size increased through clonal growth until summer, reaching a maximum of 110 shoots per tuft. The tuft size decreased during fall and winter, possibly due to shoot die-off. Therefore, the tuft dynamics of Z. caespitosa depend primarily on asexual reproduction, although sexual reproduction could potentially contribute to new tuft formation [22,29].

Seagrass can reproduce sexually through seedling recruitment via seed germination and asexually through clonal growth [30,31]. Asexual reproduction of seagrass is the primary mechanism, maintaining their population since seedling recruitment through sexual reproduction has very low in natural conditions [30]. On the other hand, the sexual reproduction of seagrass is crucial for the persistence and recovery in disturbed areas, enhancing their resilience to disturbances [32,33,34]. The sexual reproductive effort of seagrasses varies considerably, both spatially and temporally, as well as among species. Moreover, reproduction is closely related to local environmental conditions [17,31,35]. The cosmopolitan seagrass species Z. marina in temperate regions of the northern hemisphere are often characterized by low sexual reproductive effort (<10%), but the percentages of flowering shoots exceeding 50% also have been reported in some geographical regions [36,37,38]. In the present study, the sexual reproduction of Z. caespitosa was characterized by a maximum percentage of flowering shoots of 5.8%, approximately 7.1 inflorescences per shoot, and 10.9 seeds per inflorescence. The percentage of flowering shoots for Z. caespitosa at the study site (5.8%) was much lower than that in north Hokkaido (14.2–27.4%) and northeastern Honshu (13–44%), Japan [29]. Nevertheless, the number of inflorescences per shoot at the study site was similar to those in Japanese meadows and other Z. caespitosa meadows in Korean coastal waters (Yulpo Bay: 6–13.6, Duksan Port: 7–9, Jinhae: 4–9, and Bakyungdo: 5–6) [29,39].

A comparison of Z. caespitosa with Z. marina and Z. caulescens revealed a similar leaf length and width, but Z. caespitosa had a much lower number of inflorescences per shoot (7.1) and the number of seeds per inflorescence (10.9) in this study than in the other two species of previous studies [17,29]. For Z. marina in the northwestern Pacific region, the number of inflorescences per shoot ranged from 9 to 32 (mean: 15.6 inflorescences shoot⁻¹), while the number of seeds per inflorescence ranged between 3 and 13.3 (mean: 10.9 seeds inflorescence⁻¹) [17,18,19,20,21,22,23,24,25,26,27,28,29]. Z. caulescens was characterized by 10–20 inflorescences per shoot and 11.8–18 seeds per inflorescence in Japan [29]. Z. caespitosa had a lower potential for seed production in the present study site (1857 seeds m⁻²) than the other species in the genus Zostera because of the low flowering percentage, the number of inflorescences per shoot, and the number of seeds per inflorescence. The potential seed production of Z. marina ranges from 500 to 100,740 seeds m⁻² globally, while that of Z. japonica ranges between 1516 and 54,120 in the northwestern Pacific region [40,41,42,43]. Therefore, the sexual reproduction of Z. caespitosa at the study site is likely to be much lower than other species in the genus Zostera.

4.2. Factors Regulating the Growth of Zostera caespitosa

The growth of Z. caespitosa exhibited distinct seasonality in the present study. The shoot density, biomass, and productivity of Z. caespitosa increased during spring, reaching their highest point in early summer (June or July) and decreasing rapidly when the water temperature exceeded its optimal growth temperature. In addition, the areal leaf productivity, total biomass, and shoot density of Z. caespitsa were strongly correlated with the water temperature (Table 2). Seasonal variations in the biological characteristics of seagrasses are usually influenced by changes in water temperature, which is considered to be a major factor regulating the seasonality of seagrass growth [16,44,45].

Seagrasses in the northwestern Pacific are generally considered cold-affinity species [46,47]. The optimal growth temperature for Z. marina in Korea is approximately 15–20 °C, with severely inhibited growth occurring at water temperatures above 20 °C [46]. The P. japonicus productivity on the southeastern coast of Korea was highest at ~14 °C in late winter and early spring, and then decreased gradually [47]. Similarly, the highest biomass of Z. caulescens in the East Sea, Korea was recorded at around 12–14 °C [44]. The areal leaf productivity and total biomass of Z. caespitosa in this study increased until the water temperature reached ~22.5 °C, followed by a rapid decrease as the temperature increased further. The peak growth and photosynthesis for Z. caespitosa were also observed between 15 and 20 °C in indoor experiments [48]. According to the present results, the optimal growth temperature for Z. caespitosa was approximately ~22.5 °C, similar to that of Z. marina on the southern coast of the Korean peninsula. Rapid decreases in seagrass growth in summer have been observed for many temperate species [17,32,36]. The productivity, biomass, and shoot density of Z. caespitosa in this study also declined rapidly during summer, suggesting that high temperatures inhibit the growth of this species.

The underwater irradiance is considered to be a primary parameter determining the growth, survival, and distribution of seagrasses, but it was not closely related to Z. caespitosa growth at this study site [12,13]. Only the shoot density showed a significant association with the underwater irradiance in the present study. In contrast, the leaf productivity and total biomass of Z. caespitosa were not correlated with the underwater irradiance. Similarly, there was no significant correlation between the Z. marina productivity and the underwater irradiance in Kosung Bay and Koje Bay on the South Sea, Korea because the daily maximum underwater irradiance in these bays exceeded the light saturation irradiance of Z. marina (~100–200 μmol photons m⁻² s⁻¹) [12,46]. In addition, the shoot density and biomass of Z. caulescens in the East Sea, Korea were not correlated with the underwater irradiance, as the plant received sufficient light for photosynthesis [44]. On the other hand, it was presumed that the saturation irradiance of Z. caespitosa might be lower than that of Z. marina because Z. caespitosa is observed at greater depths than Z. marina [10,11]. The daily maximum irradiance in this study typically exceeded the saturation irradiance of Z. marina [12,46]. Therefore, Z. caespitosa likely received sufficient light at the study site. Despite the high fluctuations in underwater irradiance, Z. caespitosa growth showed a clear seasonal variation. Thus, the seasonal variations of Z. caespitosa were probably not controlled by the irradiance in this study.

Although surface irradiance was highest during the summer period, the underwater irradiance typically decreases severely in summer because of an increase in phytoplankton [49,50,51]. The high phytoplankton growth is associated with high surface irradiance, temperature, and precipitation during the summer [50,51]. In the present study, the underwater irradiance decreased continuously during summer when the monthly average water temperature exceeded the optimal growth temperature (~22.5 °C) from July to October. This corresponded to the rapid decline in the growth of Z. caespitosa during the same period. Because the light requirement for seagrass growth increases under high-temperature conditions, the decrease in Z. caespitosa growth during the summer period could be due to the combined effects of the reduced light and high-water temperature [12,13].

The availability of inorganic nutrients is a key factor in influencing seagrass growth [12,52]. While seagrasses have the ability to absorb inorganic nutrients from both sediment pore water and seawater, the nutrients in sediments are typically regarded as the primary sources that govern seagrass growth [20,21,53]. This is because the primary nutrients, such as NH₄⁺ and PO₄³⁻, in sediment pore water are much higher than those in seawater. In the present study, the dissolved inorganic nitrogen (DIN; NH₄⁺ and NO₃⁻+NO₂⁻) in the sediment pore water was not correlated with the growth of Z. caespitosa, except for the relationship between the areal leaf productivity and NO₃⁻+NO₂⁻ concentration. Nevertheless, the PO₄³⁻ in sediment pore water was related to the leaf areal productivity, total biomass, and shoot density. Nitrogen is generally regarded as the most limiting nutrient to seagrass growth, but there are seagrass meadows in tropical and temperate regions where phosphorus limitations prevail [52,54,55]. The growth of Z. caespitosa was strongly correlated with the PO₄³⁻ concentration in sediment pore water, suggesting the possible phosphorus limitation for Z. caespitosa at the study site.

On the other hand, inorganic nutrients in the seawater were not correlated with Z. caespitosa growth. The inorganic nutrients in seawater are usually low (<3 μM for NH₄⁺ and NO₃⁻ and <2 μM for PO₄³⁻), except in areas with a high river input [12]. The average nutrient concentrations in seawater at the study site (0.7 μM for NH₄⁺, 1.8 μM for NO₃⁻, and 0.4 μM for PO₄³⁻) were also low, which is typical for seagrass meadows. Therefore, the water column nutrient conditions were not likely the regulating variable for the seasonal growth of Z. caespitosa in this study.

5. Conclusions

The tuft density of Z. caespitosa exhibited little change throughout the year. No new tuft formation via sexual reproduction was observed in the study. However, the tuft size and biological traits within the tufts showed distinct seasonal variation, suggesting that the Z. caespitosa population depends mainly on asexual reproduction. Z. caespitosa is highly vulnerable to acute large-scale disturbance and has low resilience when its tufts are damaged because of the low sexual reproductive effort of this species. The ongoing rise in water temperature also poses a potential threat to Z. caespitosa because of the severe growth inhibition at high water temperatures. Therefore, increasing scientific knowledge and improving public awareness of these threatened seagrass species are necessary.