3. Results
3.1. Environmental Parameters
The environmental parameters tracked during the ichthyoplankton sampling sessions displayed temporal fluctuations, which are believed to play a critical role in the ecological dynamics observed. The chl-a concentrations, indicative of primary productivity, showed some fluctuations across the sampling periods. Peak concentrations were observed in April 2019 with a surface chl-a value of 3.136 mg/m3, suggesting a robust spring phytoplankton bloom. In stark contrast, markedly lower concentrations were recorded in September 2019 (surface chl-a: 0.062 mg/m3), aligning with seasonal declines in primary productivity as the region transitioned from summer to autumn.
Salinity measurements, which can influence water column stratification and habitat suitability for various marine species, also displayed variability. Notably, the salinity gradient was most pronounced in October 2018, in April 2019, and in May 2019, with a differential of −1.95, −1.64, and −1.445, respectively, between surface and deeper waters, demonstrating increased freshwater presence in the surface layer and reflecting significant vertical stratification that may limit nutrient interchange across the water column.
Temperature profiles further corroborated the presence of distinct seasonal patterns. The highest temperature gradient was observed in September 2019 (8.163 °C between surface and deeper waters), indicative of strong thermal stratification during the late summer months. Such conditions are known to affect larval dispersion and survival rates by altering the vertical distribution of their planktonic food sources.
Together, these environmental parameters—
chl-a, salinity, and temperature—provide a comprehensive picture of the biophysical conditions during each sampling event. The observed variations are crucial for understanding the seasonal and interannual dynamics of ichthyoplankton populations in the study area, as they directly influence the growth, survival, and distribution patterns of marine larvae [
10].
Table 2 provides a summarized presentation of the average values for environmental parameters recorded during each sampling season.
3.2. Ichthyoplankton Sampling
The species abundances recorded across the nine sampling events demonstrated a dynamic and varied ichthyoplankton community.
In October 2018, a moderate diversity of species was observed, with abundances of Engraulis encrasicholus (37/10 m2) and Liza ramada (26/10 m2). The spring of 2019, specifically April, showed a dramatic increase in the abundance of Engraulis encrasicholus (206/10 m2), which dominated the samples. Further, the data from May 2019 revealed an even higher spike in Engraulis encrasicholus abundance (283/10 m2), paired with numbers of Ceratoscopelus maderensis (74/10 m2). By September 2019, the individual species abundances were more balanced, with Engraulis encrasicholus at a lower yet significant 99/10 m2. The trend in 2020 continued with Engraulis encrasicholus displaying high abundances in April (383/10 m2) and May (294/10 m2). This peak diversity correlates with the highest recorded abundances of Engraulis encrasicholus (544/10 m2) and an increase in Sardina pilchardus (220/10 m2) and Spicara smaris (173/10 m2), emphasizing a significant late spring biomass accumulation.
Overall, the study identified fish larvae from 85 taxa, including 78 species across 34 families (
Table 3).
The gathered environmental data were used to assess their potential impact on ichthyoplankton distributions and abundances, employing Spearman’s rank correlation statistical method (
Table 4).
3.3. Ichthyoplankton Diversity Indices
The analysis of diversity indices (
Table 5) across the sampling events provides crucial insights into the ecological complexity and stability of the ichthyoplankton communities over the study period. These indices include Margalef, Pielou, Brillouin, Fisher, Shannon, and Simpson, each offering a different perspective on species richness and evenness.
Starting in October 2018, a moderate diversity is indicated by a Shannon index of 2.879 and a Simpson index of 0.9735, suggesting a relatively balanced ecosystem with a fair distribution of species abundances. However, it is in September 2020 that we observe a significant increase in diversity metrics, with the Shannon index reaching its highest value of 3.348 and the Simpson index at 0.9815, indicating a highly diverse and evenly distributed ichthyoplankton community. The trend of increasing diversity continues into 2020, peaking in April where the Shannon index climbs to 3.611, and the Fisher’s alpha reaches its highest at 28.48, reflecting an increase in species richness. This peak is associated with high recruitment and possibly favorable environmental conditions that support a wider range of species. By May 2021, the community structure appears even more robust, as evidenced by the highest recorded Margalef richness index for the entire study period at 11.48, alongside the highest Shannon index value of 3.887. This points to a complex ecosystem with numerous species coexisting. In contrast, Pielou’s evenness index shows less variation over time, consistently indicating a relatively even distribution of individuals among species, with values typically close to 1.0. This suggests that no single species dominates the community entirely at any sampling point, supporting the conclusion of a balanced ecosystem.
Overall, these diversity indices highlight significant temporal variability in ichthyoplankton community composition, likely influenced by seasonal cycles, environmental conditions, and interspecific interactions. This variability is crucial for understanding ecosystem health and resilience in response to natural and anthropogenic changes.
3.4. Season-Based Non-Metric Multidimensional Scaling
In this analysis, the spring cluster is formed by the sampling months of April 2020, April 2021, May 2021, and May 2020. The autumn cluster includes September 2019 and September 2020. Additionally, the months of April 2019, May 2019, and October 2018 are shown as distinct and separate from these clusters.
4. Discussion
Aquatic organisms have developed adaptive mechanisms to respond to shifts in environmental conditions, helping them maintain their normal physiological state amid real or perceived environmental changes [
11]. Factors such as deviations from natural ranges in temperature, salinity, and hydrodynamic conditions, along with food scarcity, act as stressors. These stressors challenge or disrupt the organisms’ dynamic equilibrium and significantly impact their metabolic and biochemical functions, triggering responses related to stress [
12].
Water temperature is crucial in affecting both the biochemical and physiological processes that are vital for the survival of organisms living in aquatic settings [
13]. Additionally, it determines the spawning periods of thermophilic fish species. Data from the Black Sea show a lengthening of the spawning season; anchovy ichthyoplankton, typically seen from June to September, extended their presence from May to October between 2011 and 2016 [
14]. Other significant parameters influencing the selection of spawning grounds for small pelagic fish include temperature, salinity, depth, and
chl-a levels [
15].
The study’s findings indicate interactions between environmental conditions and ichthyoplankton species dynamics across the sampling events. These interactions are evidently influenced by variations such as
chl-a, salinity, and temperature. The conditions of strong thermal stratification, particularly noted during the late summer months when the highest temperature gradient reached 8.163 °C between surface and deeper waters, are known to affect larval dispersion and survival rates [
16].
Vertical stratification, as observed in the study where freshwater layers predominantly accumulated at the surface, plays a crucial role in controlling the availability of nutrients to surface-dwelling phytoplankton. This, in turn, can significantly influence the composition and distribution of the ichthyoplankton community. The stratification patterns, characterized by limited nutrient interchange across the water column, underscore the importance of understanding these dynamics for predicting ecological responses in marine environments.
In the Litochoro area, species presence indicates fish larvae occurrences of several commercially important species. Among these, Engraulis encrasicholus (anchovy) is particularly prominent, with abundances peaking dramatically in May 2021 at 544/10 m2, underscoring its critical role both as a staple in marine food webs and in local fisheries. The spring 2019 increased abundance of Engraulis encrasicholus is indicative of a seasonal spawning event or migration pattern into the sampling area. The further rise of anchovy abundance in May 2019 points to an ongoing high productivity phase possibly linked to the warmer temperatures and increased chl-a levels observed during this period. By September 2019, the overall species diversity appeared to increase, shown by the highest recorded Shannon index value of the study period (3.348). This shift might reflect a transition in community structure after the peak reproductive season. The 2020 trend for Engraulis encrasicholus continued displaying high abundances, underscoring its critical role within the local marine ecosystem. The diversity indices and species count reached their peaks in May 2021, with the Shannon index peaking at 3.887, reflecting an exceptionally diverse community. These patterns suggest that temporal variations in species abundances are likely influenced by both abiotic factors such as temperature and chl-a concentrations and biotic factors including reproductive cycles and interspecies interactions within the ichthyoplankton community.
Other commercially valuable species include
Sardina pilchardus (sardine), which also showed significant numbers, especially in April 2020 with an abundance of 383/10 m
2. These species are key targets in local commercial fishing due to their demand in the market and culinary uses. Additionally,
Pagrus pagrus (red porgy) and
Mugil cephalus (grey mullet) consistently appeared throughout the sampling, indicating their stable presence in the ecosystem. Other species such as
Mullus barbatus,
Mullus surmuletus,
Sparus aurata,
Scomber colias,
Pomatomus saltatrix,
Spondilosoma cantharus,
Merluccius merluccius, and
Lithognathus mormyrus (
Table 6) appeared in relatively low numbers, despite their significant economic importance in local fisheries [
2]. These species showed sporadic appearances across different sampling periods, often coinciding with specific environmental conditions such as variations in surface
chl-a, salinity, and temperature gradients. For instance, in April 2021,
Mullus barbatus and
Pomatomus saltatrix were noted in minimal amounts during a period characterized by intermediate
chl-a concentrations and slight variations in temperature and salinity gradients, which indicate the water column stability. This pattern suggests that larger environmental shifts, particularly in primary productivity (as indicated by
chl-a) and water column stability (as suggested by salinity and temperature gradients), could influence the distribution and spawning activities of these species. Moreover, larval distribution is heavily influenced by sea currents that play a crucial role in the distribution of marine larvae, influencing their dispersal across various habitats and impacting the broader marine ecosystem dynamics, facts that could also explain their lower-than-expected appearance in this study’s samples [
17].
Τhe Spearman analysis showed that high anchovy abundances coincide with low average chlorophyll values (R = −0.65,
p < 0.05) (
Table 6). In May 2021,
chl-a levels reached 0.351 mg/m
3 at the surface, coinciding with peak abundances of
Sardina pilchardus and
Spicara smaris (220/10 m
2 and 173/10 m
2, respectively). This period also reflects some level of primary productivity; however, the correlation is not statistically significant (R = −0.069–0.42,
p > 0.05). It appears that other oceanographic parameters are more crucial for the above species abundances (
Table 6).
The relatively higher chl-a levels observed in October 2018, recorded at 0.967 mg/m3, indicate enhanced primary productivity, which could support a diverse array of marine species. Similar to anchovy, the Spearman analysis demonstrated a significant but negative trend for A. laterna regarding chl-a values (R = −0.69, p < 0.05), and for the majority of species the correlations were negative or insignificant (R = −0.61–0.36, p > 0.05). However, the lowest recorded salinity gradient of −1.956 during the same period could influence the vertical distribution of nutrients and organisms, potentially enriching surface waters where most photosynthesis occurs.
The salinity gradient noted in May 2020 shows a minor connection with the variety of species present. As salinity can significantly influence the distribution of marine larvae [
18], changes in this parameter likely impact the habitat suitability for species such as
Chelon ramada and
Mugilidae, which were abundant during this period. The abundances of these species showed a good interrelationship with the salinity gradient, yet insignificant as demonstrated by Spearman analysis (R = 0.5,
p = 0.12 for both). The factor that seems to play a crucial role in predicting the abundances of species is either surface salinity (
E. encrashicolus: R = 0.71,
A. laterna: R = 0.87,
Gobius spp.: R = 0.74,
p < 0.05) or average salinity (
S. pislchardus: R = 0.78,
S. smaris: R = 0.86) (
Table 6), indicating that their presence is more enhanced with minimum freshwater outputs, pointing at the summer/autumn period. Conversely, high river runoffs increased the abundances of
P. acarne (R = −0.68,
p < 0.05) and
S. porcus (R = −0.76,
p < 0.05).
Additionally, in the Litochoro area, temperature is a significant factor for the presence of S. porcus (average T: R = 0.48, surface T: 0.70, p < 0.05), indicating high abundances during the summer and early autumn period and suggesting the opposite trend for S. smaris (average T: R = −0.88, surface T: −0.73, p < 0.05). For S. colias, the most significant parameter for its presence is the temperature gradient (R = 0.79, p < 0.05), being more favored under strong thermal stratification.
High diversity values, such as those seen in May 2021 (Shannon index 3.887), indicate a stable and resilient community. This diversity supports a variety of trophic interactions and ecological roles, essential for maintaining ecosystem health. The presence of both high and low abundance species like
Engraulis encrasicholus and
Callionymus lyra across different sampling points suggests a dynamic balance influenced by ongoing environmental changes. Interestingly, most diversity indices, apart from Pielou’s normality index (J) and Simpson’s diversity index (1–lambda), show a significant positive correlation with surface salinity. This suggests that increased salinity levels might boost certain facets of biodiversity during the summer and early autumn months. Stable environmental conditions, such as consistent higher salinity and warmer temperatures, indeed contribute to the establishment of stable breeding and nursery grounds for fish. These conditions can enhance the recruitment success of fish species by modulating their growth, condition, and survival. Coastal areas, which include these nursery grounds, are essential for the development of juvenile fish and are highly dependent on the environmental quality of these areas. The quality and stability of these habitats are crucial for supporting diverse and healthy populations of juvenile fish, which are key to sustaining fish populations overall [
19].
A related study on ichthyoplankton assemblages was conducted in the marine protected area of the artificial reef of Kitros, located north of Litochoro [
3]. By comparing the two, key differences emerge primarily due to Kitros’s greater river influence. Kitros exhibits significant fluctuations in salinity and temperature, indicating freshwater inflow, which impacts its marine conditions. This is in contrast to Litochoro, where more stable marine conditions prevail, showing moderate seasonal temperature variations and relatively stable salinity levels.
Chl-a levels are consistently higher in Kitros [
3], likely due to nutrient-rich river runoff, enhancing primary productivity. This contributes to a higher variability and abundance of species in Kitros, including riverine species and those tolerant to lower salinity. In contrast, Litochoro’s species composition is more consistent, dominated by species like
Engraulis encrasicholus, suggesting favorable marine conditions for spawning. Diversity indices in Kitros show more fluctuation [
3], reflecting the dynamic interaction between freshwater and seawater, whereas Litochoro’s indices suggest a stable and diverse ecosystem. The differences highlight the need for tailored conservation strategies that consider the distinct ecological characteristics of each area.
As far as the season-based MDS analysis (
Figure 2) is concerned, the clustering of April and May in 2020 and 2021 can be attributed to similar species abundances influenced by recurring spring ecological conditions. During these months, favorable environmental factors such as increasing temperatures and a rich supply of planktonic food promote high reproductive activities among key species. Specifically, in April 2020, the surface temperature reached 21.446 °C with a temperature gradient of 7.657 °C, indicative of warmer surface waters that can enhance plankton growth. October 2018, the only sampling to take place so late in autumn, is distinct for having a relatively low abundance of
Engraulis encrasicholus at 37/10 m
2, which is notably lower compared to that in other months. This contrasts with April 2019 and May 2019 where the abundance of anchovies was considerably higher at 206/10 m
2 and 283/10 m
2, respectively. However, what further differentiates April and May from other periods are the additional species with significant abundances. April 2019 featured counts of
Diplodus sargus and
Dilpodus annularis, each at 24/10 m
2, and
Spicara smaris at 69/10 m
2. May 2019 continued this trend with
Ceratoscopelus maderensis at 74/10 m
2 and
Spicara smaris at 52/10 m
2. These specific peaks in species other than anchovies contribute to their ecological distinctiveness, influencing the patterns. This result is strengthened by the fact that these months are characterized by higher river presence (as indicated by higher surface salinity and a lower salinity gradient) and, as a result, by higher primary production (as indicated by higher surface and average
chl-a values).
Engraulis encrasicholus, a pivotal species for indicating spring productivity, showed increases in numbers with abundances reaching 383/10 m
2 in April 2020 and an even higher 544/10 m
2 in May 2021, with both months being characterized by high temperature gradients (
Table 2). This pattern is indicative of its spawning period, which typically peaks during spring [
20]. Similarly,
Chelon ramada exhibited high abundances during these months. Other species such as
Sardina pilchardus and
Spicara smaris displayed increased numbers in the April and May 2021 samplings. This consistency led to the grouping of these specific months in the multivariate statistical analysis, highlighting a stable seasonal pattern in the community structure of ichthyoplankton in the region. The clustering of September 2019 and September 2020 suggests that these months shared similar dynamics, as during September, the marine environment begins to shift from summer to autumn, affecting water temperatures, nutrient dynamics, and species’ behaviors [
21]. In September 2019 and September 2020, specific species such as
Engraulis encrasicholus showed significant abundances, with counts of 99/10 m
2 and 299/10 m
2, respectively. Other species like
Gobius spp. [
22] and
Scorpaena porcus [
23] also displayed consistent patterns between these months, with
Gobius spp. at 116/10 m
2 in September 2019 and similar levels in 2020.
These similarities in species abundances are influenced by several factors typical of September in the region [
3]. The water temperatures are generally beginning to cool but remain warm enough to support active feeding and growth for many species. Additionally, the post-summer period might still benefit from residual summer productivity, providing sufficient food resources for a diverse range of species. The regular occurrence of similar species abundances during these months of consecutive years likely results in their grouping in the multivariate analysis, demonstrating a consistent ecological pattern driven by the seasonal transition from summer to autumn in the Litochoro marine environment.
These observations underline the critical need for continuous monitoring to adaptively manage ichthyoplankton populations and the broader marine ecosystem. The association between environmental variables and species abundances illustrates how climate change might influence marine biodiversity [
24]. Extended monitoring and more detailed spatial analysis are recommended to refine the understanding of the observed patterns. Investigating the genetic diversity within populations, especially for highly abundant and ecologically significant species like
Engraulis encrasicholus and
Sardina pilchardus, could reveal insights into their resilience and adaptability to environmental variability.
5. Conclusions
This study has described the dynamics between environmental factors and the community structure of ichthyoplankton in the Litochoro area, underscoring the influence of abiotic factors such as chl-a, salinity, and temperature on marine species’ distribution and abundance.
In the coastal region of Litochoro, surface salinity significantly influences the distribution of ichthyoplankton species such as Engraulis encrasicholus, Arnoglossus laterna, and Gobius spp., while average salinity is more critical for Sardina pilchardus and Spicara smaris. These findings suggest that lower freshwater outputs, typically occurring in the summer and autumn, enhance the presence of these species. Conversely, species like Pagellus acarne and Scorpaena porcus thrive with higher river runoffs. Additionally, temperature plays a significant role in the abundance of Scorpaena porcus, showing peak numbers in summer and early autumn, while Spicara smaris exhibits a reverse trend. For Scomber colias, the most significant factor affecting its distribution is the temperature gradient, with greater abundance under conditions of strong thermal stratification. Anchovy, key for indicating spring productivity, significantly increases in abundance during the spring, aligning with its peak spawning period. This pattern underscores the critical influence of temperature gradients during its reproductive timing.
Anchovy, the most abundant species in this study, exhibits a negative correlation with high average chl-a values, indicating that high abundances of anchovy coincide with low average chlorophyll values. Furthermore, most diversity indices display a strong negative correlation with high chl-a values, suggesting that higher concentrations of chl-a may be linked to lower biodiversity. However, it is well documented that fish larvae can only consume phytoplankton indirectly; the phytoplankton must first be eaten by zooplankton, which in turn affects fish populations. In contrast, most diversity indices show a significant positive correlation with surface salinity. This indicates that increased salinity levels may enhance certain aspects of biodiversity during the summer and early autumn months. This pattern has been consistent over multiple years, demonstrating the species’ dependence on stable environmental conditions to sustain its population dynamics.
More stable marine conditions prevail compared to other previously studied regions in the gulf of Thermaikos. The area shows moderate seasonal temperature variations and relatively stable salinity levels. Litochoro’s species composition is consistent, dominated by species like Engraulis encrasicholus, suggesting favorable marine conditions for spawning. Litochoro’s diversity indices suggest a stable and diverse ecosystem. More stable marine conditions in Litochoro support a unique community structure, less influenced by riverine inputs and more by marine stability. This contrast underscores the necessity for region-specific conservation and management strategies that cater to the unique environmental and ecological conditions of each area.
In conclusion, the findings from this study advocate for continued and detailed monitoring of the marine ecosystem in the Litochoro area. Such efforts are vital for understanding the ongoing changes and potential future shifts in ichthyoplankton communities due to environmental pressures, including climate change. Furthermore, understanding the genetic diversity within key species could provide deeper insights into their resilience and adaptive capacities, essential for developing effective management and conservation strategies. This comprehensive approach will help safeguard the ecological and economic viability of these important marine resources.