Bacterioplankton Community Structure and Its Relationship with Environmental Factors in the Coastal Waters Around the Changli Gold Coast National Nature Reserve in Northern China

Abstract

Marine bacterioplankton perform a very important role in the cycles of carbon, nitrogen, phosphorus, and other elements in coastal waters. The impacts of environmental factors on bacterial community structure are dynamic and ongoing. This study investigated the spatiotemporal distributions of elements and their influences on bacterioplankton communities in the coastal waters around the Changli Gold Coast National Nature Reserve in northern China. The results demonstrate the significant temporal variability of phosphorus, nitrogen, and carbon in spring and summer, influenced by natural environmental factors and anthropogenic activities. In spring, increased biological activity, particularly phytoplankton growth, may elevate TOC and POC levels near the river estuaries, while in summer, microbial decomposition likely stabilized carbon concentrations. The seasonal variation in the bacterioplankton community was obvious. Bacteroidetes were enriched in spring samples and Cyanobacteriota proliferated in summer. The dominated genera in the spring, including Planktomarina, an unclassified NS5_marine_group (belonging to Flavobacteriaceae), and the OM43_clade (Methylophilaceae), showed significant positive correlation with salinity, TDP, TOC, POP, and DO levels, while Synechococcus_CC9902 (Synechococcus), PeM15_unclassified (Actinobacteria), and HIMB11 (Rhodobacteraceae), which all dominate in summer samples, are significantly positively correlated with TN, TDN, temperature, and ammonium levels. In summer in particular, the increase in human activities and river inputs greatly improves nutrient levels and promotes the propagation of photosynthetic microorganisms. These results indicate that the nutrient elements and environmental physical conditions are affected by seasonal changes and human activities, which have significant effects on the community structure of bacterioplankton. This study highlights the importance of ongoing monitoring in estuarine coastal areas, especially in protected areas like the Changli Reserve, to manage eutrophication risks and maintain ecological balance.

1. Introduction

Human activities, including atmospheric emissions of carbon dioxide from fossil fuels, the application of agricultural fertilizers, and the discharge of industrial and municipal sewage into rivers, are significant contributors to the elevated levels of phosphorus, nitrogen, and carbon in offshore waters. The combined effects of nutrient loading and climate change are expected to further impact ocean productivity, the cycling of marine elements, and the structure and function of marine ecosystems [1]. Additionally, ocean deoxygenation driven by human activities presents a serious challenge to marine ecosystems, an issue that has not received adequate attention to date [2,3]. Therefore, in the context of climate change, it is increasingly important to study the effects and interactions of these three key drivers, including nutrient loading and ocean deoxygenation. Estuary inflow and submarine groundwater inflow are the two main ways that nitrogen and phosphorus sources enter the marine ecosystem from land, resulting in a very high degree of eutrophication, which also has a significant impact on water quality and the ecosystem [4]. Pollutants discharged into the ocean through different estuaries have significant and varied effects on the microbial community [5,6,7,8,9]. Those pollutants often include heavy metals, organic pollutants, and nutrients [10,11,12]. At the same time, the presence of these pollutants changes the structure and function of the microbial community, thus affecting the entire marine ecosystem.

Microorganisms, especially bacterioplankton, as key participants in biogeochemical cycles, play a vital role in maintaining the balance of marine ecosystems [12,13,14,15], particularly in coastal areas. Coastal areas are the link and bridge between land and sea and exhibit complex biogeochemical processes [16,17]. The composition and distribution of microbial communities in this region are significantly affected by the in situ environment. During outbreaks of red tides, microorganisms and algae in the ocean interact closely and jointly respond to changes in the marine ecosystem [18,19]. During special periods of flowering, specific functional flora increase. For example, the content and structure of soluble polysaccharides produced through plankton algae reproduction have an important impact on the community structure of plankton bacteria and also enrich the groups of degradable polysaccharides [19]. These microorganisms can be used as biological indicators of red tide occurrence [18,19].

The Bohai Sea, located in China, is a highly urbanized and rapidly developing coastal area. Up until 2012, its development had led to water pollution issues, primarily due to the influx of inland pollutants and the influence of local hydrogeological conditions. In particular, the western part of Bohai Bay faced particularly severe pollution problems due to its poor water-exchange capacity [20]. In the past ten years, the Chinese government has made great efforts to protect the environment and adopt constructive measures, which have significantly improved the pollution problem in the Bohai Bay area. However, the pollution problem is still not to be underestimated [4]. For example, during the summer months, increased anthropogenic activities, such as tourism, contribute to elevated concentrations of inorganic nitrogen and reactive phosphorus in coastal waters. These conditions often lead to harmful algal blooms, which have a detrimental impact on the coastal environment and economic activities.

This paper aims to investigate the variation characteristics and spatial distribution patterns of the bacterioplankton community and the phosphorus, nitrogen, and carbon levels in the coastal water during spring and summer in the Changli Gold Coast National Nature Reserve in northern China. The research results can provide a theoretical basis for management and protection measures for coastal estuaries in China.

2. Materials and Methods

2.1. Study Area and Sample Collection

The sampling area was focused on the coastal estuarine waters extending from the Daihe Estuary to the Luanhe Estuary in Hebei Province, northern China (Figure 1). This region encompasses a series of dynamic and ecologically sensitive zones that are influenced by both natural processes and anthropogenic activities. The Daihe River traverses the renowned Agricultural Dream Kingdom, characterized by extensive agricultural lands dedicated to picking, which are longitudinally aligned along the river. This river is subject to impacts from agricultural runoff and tourism activities, particularly at the Daihe Estuary, located in Beidaihe.

To the west of the Nandaihe area lies the Yanghe River, which originates from the Yanghe Reservoir and discharges into the ocean via the Yanghe Estuary. The coastal stations situated between the Daihe and Yanghe Estuaries generally exhibit relatively clean water quality, as there are no aquaculture facilities in this region. However, from the southern boundary of the Yanghe Estuary to the Luanhe Estuary, aquaculture operations have been established. Notably, this area includes the Changli Gold Coast National Reserve, where aquaculture is prohibited. Despite this restriction, a limited number of aquaculture facilities continue to operate within the buffer zone. Beyond the reserve’s boundaries, extensive aquaculture infrastructure is present.

To obtain a comprehensive overview of the environmental conditions, a total of 21 sampling stations (S1 to S21) were established throughout the designated study area. Samples were collected during the spring (April) and summer (August) of 2021, yielding a dataset of 42 samples. Each sampling event involved the collection of seawater at a depth of 0.5 m (surface) and 5 m (bottom) using 2 L sterilized bottles. The systematic labeling of samples was tailored to reflect specific locations and sampling times, thereby facilitating detailed spatial and temporal analyses.

2.2. Determination of Environmental Parameters

Surface samples from each station were used to detect environmental factors. The environmental parameters were measured following the protocols established in our previous study [21]. The detailed procedures are outlined as follows. A total of 500 mL of seawater from each sample was transferred into a polyethylene bottle and mixed with 50% sulfuric acid solution. The mixture was then thoroughly mixed and stored at −20 °C for the determination of TN, TP, and TOC levels.

Physical parameters of the seawater, including salinity, temperature, depth, and pH, were measured using a portable YSI Pro Plus multiparameter instrument (YSIInc., Yellow Springs, OH, USA) in situ. Measurements of other essential physicochemical parameters, such as dissolved oxygen (DO), phosphate (PO₄³⁻), ammonium (NH₄^“), nitrate (NO₃⁻), and nitrite (NO₂⁻) (also referred to as available phosphorus) levels, were taken under the “Specification for Oceanographic Survey” (GB/T 12763.4-2007) in the laboratory.

In the lab, a cumulative volume of 1350 mL of seawater was collected from each sample and subsequently filtered through 0.45 µm mixed cellulose ester filters. These filters were preconditioned by soaking in a 1% hydrochloric acid solution for 12 h, followed by thorough rinsing with purified water until neutrality was achieved, and were then stored in distilled water. The filtrate was transferred into a container and the concentrations of dissolved organic carbon (DOC), total dissolved phosphorus (TDP), and total dissolved nitrogen (TDN) were measured. Additionally, the material retained on the filters was utilized to quantify particulate organic phosphorus (POP) and inorganic nitrogen (PIN).

Quantification of DOC and total organic carbon (TOC) was performed using a specialized total organic carbon analyzer, while the analysis of TDP, TDN, total phosphorus (TP), and total nitrogen (TN) adhered to the guidelines outlined in the “Specification for Oceanographic Survey” (GB/T 12763.4-2007). For the examination of PIN and POP, the filters were immersed in digestion vials containing 25 milliliters of 0.1 mol/L hydrochloric acid and agitated for 2 h. The supernatant was subsequently isolated for PIN analysis, and the remaining filter served as the basis for POP analysis, following the methodology described by Zhou et al. [22]. For POP determination, the remaining filter was placed in a polyethylene vial with 25 mL of clean water and 2.5 mL of potassium persulfate, subjected to autoclave digestion for 30 min, and cooled to ambient temperature before analysis using the TP method. Particulate nitrogen (PN) was calculated by deducting TDN from TN, and particulate organic nitrogen (PON) was determined by subtracting PIN from PN, while particulate organic carbon (POC) was determined by subtracting TOC from DOC.

2.3. High-Throughput Sequencing and Data Analysis and Statistics

After recovery, the seawater (surface and bottom) was immediately filtered with a 0.22 μm polycarbonate member (Millipore, Burlington, MA, USA). Total DNA was extracted and purified with the E.N.Z.A.TM Water DNA Kit D5525-01 (Omega, Macon, GA, USA) according to the manufacturer’s protocol. DNA concentrations were measured using a NanoDrop 2000 spectrophotometer (Thermo Scientific). The hypervariable V3–V4 region of the bacterial 16S rRNA gene was amplified using the primer pair of 341F (5′-CCTACGGGNGGCWGCAG-3’) and 805R (5’-GACTACHVGGGTATCTAATCC-3’). The PCR procedure was taken from previous articles [21]. High-quality sequences were extracted from the raw Illumina reads and sorted into individual samples according to their unique barcodes. All the primers and barcodes were removed to obtain Fastq format files. Further analysis was conducted with the QIIME standard pipeline (version 1.9.1) [23]. Then, the high-quality sequences were de-noised using the DADA2 plugin [24] with recommended parameters, and amplicon sequence variants (ASVs) were obtained. The taxonomic annotation of ASVs was based on the SILVA SSU 138 rRNA database.

2.4. Data Analysis and Statistics

To visually assess the spatial distribution characteristics of various seawater environmental factors during the spring and summer seasons, we utilized Surfer 15 software to generate contour maps.

Analysis of α and β diversity based on ASV feature sequences was carried out to assess species complexity within and between taxa. We analyzed the community richness, Shannon diversity, and Pielou’s evenness. We used one-way ANOVA and Duncan’s multiple range test to compare and analyze the microbial communities with p < 0.05 determined as the significance level. The detailed statistical analysis of the data was carried out in line with methods from previous articles [21].

3. Result

3.1. Spatiotemporal Distributions of Basic Physical Environment Factors

To determine the spatiotemporal distributions of basic environment factors as well as phosphorus, nitrogen, and carbon levels in coastal waters around the Changli Gold Coast National Nature Reserve in northern China, nineteen factors were measured in 42 seawater samples. On the temporal scale, DO values ranged from 8.66 to 10.80 mg/L in spring (mean 9.68 ± 0.50 mg/L) and from 5.22 to 11.00 mg/L in summer (mean 7.88 ± 1.75 mg/L) across different stations. The pH ranged from 8.00 to 8.24 in the spring (mean 8.12) and 7.86 to 8.23 in the summer (mean 8.10). Temperature ranged from 9.00 to 12.60 °C in spring (mean 11.08 °C) and 24.10 to 26.80 °C in summer (mean 25.42 °C). Salinity ranged from 32.15‰ to 33.03‰ in spring (mean 32.51‰) and 28.76‰ to 30.95‰ in summer (mean 29.83‰). The spatial distributions of these four factors are different in spring and summer (Figure 2). The high DO values in spring were mainly distributed across the limited coastal areas of the Dapu Estuary, while the high DO values in summer covered a larger coastal area from south of the Yanghe Estuary to north of the Xinkaikou Estuary (Figure 2A,E). The distribution of pH showed a different pattern. In spring, the coastal waters surrounding the Daihe Estuary, to the south of the Xinkaikou Estuary, and the core area of the Changli Protected Area showed high pH values (Figure 2B,F). However, the high-pH areas in summer shifted to the offshore sea area. The high-temperature areas in spring were mainly distributed around coastal stations and decreased with the increase in offshore distance (Figure 2C,G). The high-temperature stations in summer were mainly distributed between the Yanghe Estuary and the Dapuhe Estuary. In both spring and summer, the high-salinity areas were distributed around the offshore stations. Salinity levels were lower at nearshore stations (Figure 2D,H). In addition, the area of high salinity in spring is limited, while the area of high salinity in summer is distributed across a wider area near offshore stations.

3.2. Spatiotemporal Distributions of Different Forms of Carbon

In this study, we measured three different forms of carbon, namely TOC, DOC, and POC. TOC ranged from 4.20 to 11.76 mg/L in the spring (mean 6.75 mg/L) and 4.40 to 5.46 mg/L in the summer (mean 4.81 mg/L). DOC ranged from 2.72 to 4.33 mg/L in the spring (mean 3.54 mg/L) and from 3.32 to 4.02 mg/L in the summer (mean 3.60 mg/L). POC ranged from 0.53 to 8.37 mg/L in spring (mean 3.21 mg/L) and 0.75 to 1.88 mg/L in summer (mean 1.21 mg/L). The distributions of the three different forms of carbon varied in spring and summer. In general, both TOC and POC levels were higher in spring than in summer, and the spatial distribution characteristic of TOC and POC was similar in the two seasons. However, the distribution of TOC and POC in two different seasons was inconsistent. Specifically, in spring, the high-value areas of TOC and POC were distributed across both the nearshore and offshore waters around the Luanhe Estuary, as well as at S18 (Figure 3A,B) and at offshore stations in the Dapuhe Estuary. In summer, the high-value areas of TOC and POC were distributed around the nearshore station S17 (Figure 3D,E) between the Yanghe Estuary and the Dapuhe Estuary. The spatial distribution of DOC was different in spring and summer. The high-value areas of DOC in spring were mainly distributed around station S17 between the Yanghe Estuary and Dapuhe Estuary and the intermediate station S5 near the Luanhe Estuary (Figure 3C). The summer high-value areas for DOC were mainly distributed around station S20 between the Daihe and Yanghe Estuaries, the nearshore stations between the Xinkai and Luanhe Estuaries, and the offshore station S6 (Figure 3F).

3.3. Spatiotemporal Distributions of Different Forms of Nitrogen

In this study, we measured the concentrations of different forms of nitrogen, including TN, PN, PIN, PON, TDN, nitrate, nitrite, and ammonium salts (Figure 4). Overall, TN ranged from 116.33 to 214.67 μg/L in spring (mean 155.05 μg/L) and from 203.56 to 360.22 μg/L in summer (mean 266.57 μg/L). TDN ranged from 79.67 to 155.78 μg/L in spring (mean 120.25 μg/L) and from 174.00 to 306.22 μg/L in summer (mean 222.28 μg/L). PN ranged from 5.00 to 73.33 μg/L in spring (mean 34.80 μg/L) and from 17.33 to 97.33 μg/L in summer (mean 44.29 μg/L). PIN ranged from 0.27 to 9.17 μg/L in the spring (mean 2.50 μg/L) and from 0.80 to 9.30 μg/L in the summer (mean 3.77 μg/L). PON ranged from 3.55 to 73.06 μg/L in spring (mean 32.30 μg/L) and from 13.92 to 92.74 μg/L in summer (mean 40.52 μg/L). Nitrates ranged from 5.28 to 61.80 μg/L in spring (mean 24.58 μg/L) and from 24.10 to 147.00 μg/L in summer (mean 77.43 μg/L). Nitrite ranged from 0.96 to 18.60 μg/L in spring (mean 3.25 μg/L) and from 2.41 to 26.20 μg/L in summer (mean 11.58 μg/L). Ammonium salts ranged from 6.67 to 46.40 μg/L in spring (mean 24.13 μg/L) and from 22.80 to 70.10 μg/L in summer (mean 45.53 μg/L). TIN ranged from 28.94 to 97.50 μg/L in spring (mean 51.96 μg/L) and from 56.08 to 226.40 μg/L in summer (mean 134.54 μg/L).

Except for PIN, all other forms of nitrogen salt content were higher in summer than in spring (Figure 4). The high-value areas of PIN were similar in spring and summer, and the high concentration levels of PIN in spring were mainly distributed around the offshore station in the Dapu Estuary (S19) and the nearshore station in the Luanhe Estuary (S3) (Figure 4D). The high-value zone of PIN in summer was mainly distributed along the cross-section between the Daihe Estuary and the Dapuhe Estuary and was minimally affected by offshore distance (Figure 4H). The distributions of TN, PN, and PON were similar in the two seasons, and the high-value areas in spring were all distributed across the northern part of the study area, that is, the sea area between the Daihe Estuary and Dapuhe Estuary (Figure 4A–C). The high-value areas of the three nitrogen forms in summer were mainly distributed around stations S20 and S21 between the Daihe Estuary and Yanghe Estuary, and station S4 near the Luanhe Estuary (Figure 4E–G). In addition, the buffer station S14 and the core zone station S12 also had high concentrations of PN and PON (Figure 4F,G) in summer. The high-value areas of TDN were mainly distributed across the buffer zone and core area of Changli Reserve in the Dapuhe Estuary in spring (Figure 4I). In summer, they were mainly distributed around nearshore stations near the Yanghe Estuary and the Luanhe Estuary (Figure 4M). The distribution characteristics of nitrate and nitrite were similar in summer, with high values mainly distributed across the coastal area south of the Dapuhe Estuary and the vertical section of the Luanhe Estuary (Figure 4N,O). In spring, the high-value area of nitrate was mainly located in the middle of the offshore area between the Xinkaikou Estuary and the Luanhe Estuary (S9), which partially coincided with the buffer zone of the Changli Protected Area (Figure 4J). The nitrite concentration was unusually prominent at station S9 (Figure 4K). However, the high-value areas of ammonium salt in spring and summer were located outside of the Changli Protected Area, and the level of ammonium salt was low in the buffer zone and core area of the protected area (Figure 4L,P). In general, in the core area of Changli National Reserve, TDN was at its highest level in summer, TN and PON were at a higher level in spring and summer, and other nitrogen content indexes were at a lower level in spring and summer.

3.4. Spatiotemporal Distributions of Different Forms of Phosphorus

The spatial distributions of different forms of phosphorus in spring and summer were very different (Figure 5). Overall, AP ranged from 3.88 to 12.80 μg/L in spring (mean 8.23 μg/L) and from 0.00 to 6.91 μg/L in summer (mean 3.31 μg/L), respectively. TDP ranged from 6.97 to 15.35 μg/L in spring (mean 10.14 μg/L) and from 3.73 to 12.29 μg/L in summer (mean 6.56 μg/L). POP ranges from 2.87 to 5.86 μg/L in spring (mean 4.21 μg/L) and from 1.11 to 4.14 μg/L in summer (mean 2.30 μg/L). The high-value areas of POP in spring were mainly distributed around nearshore stations, except for S20, with the highest concentration measured at station S14 (Figure 5A). The high-value areas of POP in summer were mainly distributed around nearshore stations between the Daihe Estuary and Dapu Estuary, and between the Xinkai Estuary and the Luanhe Estuary (S1, S3, and S7) (Figure 5D). The high-value areas of TDP were mainly distributed around offshore stations in spring. In summer, they were mainly located at the nearshore sites between the Daihe and Xinkai Estuaries and the slightly offshore sites (S5, S4, and S8) between the Xinkaikou and Luanhe Estuaries (Figure 5B and Figure 4E). In spring, AP concentration was lower at nearshore stations and higher at offshore stations in the core area, with the exception of station S12. In particular, at stations southeast of the study area (i.e., S2, S5, S6, and S9), AP showed extremely high concentrations (Figure 5C). The highest levels of AP concentration in summer were found in the sections around the Tanghe Estuary (stations S20 and S21) (Figure 5F), followed by the Changli Protected Area and the area of sea to its south; however, the AP concentration was lower in a certain range of the sea area north of Changli Protected Area (Figure 5F).

3.5. Bacterioplankton Diversity

In this study, we have successfully sequenced and analyzed over 40,000 ASV characteristic sequences. By performing principal component analysis (PCA) using the Bray–Curtis distance, we identified the primary factor influencing the spatiotemporal distribution of bacterioplankton in the region between Daihe Estuary and Luanhe Estuary in Hebei Province, northern China (Figure 6A). Our analysis segregated the samples into two distinct groups: spring and summer. The significant separation between these two clusters indicates a substantial difference in the composition of the bacterioplankton community between spring and summer. Furthermore, our results revealed that the summer samples exhibited significantly higher Shannon diversity index and Pielou’s evenness index values compared to the spring samples (p < 0.05) (Figure 6B,C). This finding underscores that species diversity and evenness were greater during summer than during spring.

3.6. Bacterioplankton Communities

The dominant phyla of bacterioplankton are Proteobacteria, Cyanobacteriota, Actinobacteria, and Bacteroidetes (Figure 7A). However, the proportions of Cyanobacteriota, Bacteroidetes, and Actinomycetes changed significantly in two different seasons (Figure 7A). The proportion of Proteobacteria in different seasons was relatively stable (Figure 7A). Proteobacteria had the highest proportion in spring samples, accounting for up to 50% in bottom water samples (Figure 7A). Cyanobacteriota is most abundant in the summer, reaching levels of 27% to 40% (Figure 7A). The Bacteroides group was significantly enriched in spring, accounting for 28% in spring bacterioplankton communities (Figure 7A).

At the genus level, the microorganisms in the spring samples mainly include Stappiaceae_unclassified, Saprospiraceae_unclassified, Cyanobium_PCC.6307, Actinobacteria_unclassified, Cyanobacteriota_unclasslfied, Synechococcus_CC9902, PeM15_unclassified, Candidatus_Actinomarina, AEGEAN-169_marine_group_unclassified, HIMB11, and the OM60(NOR5) clade (Figure 7B). The main microorganisms enriched in summer samples included NS9_marine_group_unclassified, SAR86_clade_unclassified, Alphaproteobacleria_unclassified, Cryomorphaceae_unclassified, Candidatus_Puniceispirillum, Pseudohongiella, Bacteroidetes_unclassified, Planktomarina, Formosa, Ns3a_marine_groupSAR92_clade, Persicirhabdus, Candidatus_Aquiluna, OM43_clade, RS62_marine_group, NS5_marine_group, and Clade_la (Figure 7B). The difference in bacterial groups in spring and summer samples was more significant, indicating that the season is the key factor affecting the composition of plankton bacteria.

The results in Figure S1 show that seasonal changes have significant effects on the bacterioplankton communities of coastal water. The taxa belonging to the phyla of Fusobacteriota, Bacteroidetes, Verrucomicrobia, and Patescibacteria clustered in spring. However, there were significant differences in microbial community structure between summer and spring, with Cyanobacteriota, NB1-j, Spirochaetota, SAR324_clade, Gemmatimonadota, Marinimicrobia (SAR406 clade), Chlamydiae, PAUC34f, Bdellovibrionota, Myxococcota, and Planctomycetes present in summer (Figure S1A). At the genus level, Cryomorphaceae_unclassified, Pseudohongiella, Candidatus_Puniceispirillum, Bacteroidetes_unclassified, Planktomarina, Formosa, NS3a_marine_group, SAR92 clade, Persicirhabdus, Candidatus_Aquiluna, OM43 clade, RS62_marine_group, NS5_marine_group, and Clade_Ia were enriched in spring (Figure S1B), while in the summer, Stappiaceae_unclassified, Saprospiraceae_unclassified, Cyanobium_PCC-6307, Actinobacteria_unclassified, Cyanobacteriota_unclassified, Synechococcus_CC9902, PeM15_unclassified, Candidatus_Actinomarina, AEGEAN-169_marine_group, HIMB11, and the OM60 (NOR5) clade were enriched (Figure S1B). The genera MS9_marine_group, SAR86_clade_unclassified, and Alphaproteobacteria_unclassified were all present in both spring and summer samples (Figure S1B).

3.7. Relationship Between Distribution of Bacterioplankton Communities and Environmental Factors

To investigate the relationship between bacterioplankton and their environment, we analyzed the basic ecological parameters of seawater, the concentrations of various phosphorus, nitrogen, and carbon forms in the samples, and the correlation between the abundance of dominant bacterioplankton species and environmental factors from the surface seawater. The results indicated that several dominant phyla and genera exhibited strong correlations with other environmental factors (Figure 8). Specifically, in spring samples, there was a notable enrichment of Bacteroidetes, Verrucomicrobia, Proteobacteria, and unclassified high-abundance taxa. These taxa demonstrated a significant positive correlation with salinity, DO, POP, and TOP (p < 0.05), while exhibiting a significant negative correlation with temperature, ammonium, nitrite, and nitrate (Figure 8A). Conversely, in summer samples, Cyanobacteriota, Planctomycetes, and Actinobacteria were dominant and showed a significant positive correlation with temperature, ammonium, nitrite, and nitrate levels (p < 0.05) but had a significant negative correlation with salinity, DO, POP, and TOP (Figure 8A).

The dominant genus Planktomarina, the unclassified NS5_marine_group, and the OM43_clade in the spring samples showed significant positive correlations with salinity, TDP, TOC, POP, and DO levels, while Synechococcus_CC9902, PeM15_unclassified, and HIMB11, which all dominated in summer samples, were significantly positively correlated with TN, TDN, temperature, and ammonium levels (Figure 8B).

4. Discussion

4.1. Potential Causes Related to the Spatiotemporal Dynamics of Carbon, Nitrogen, and Phosphorus

The environmental status in the study area exhibited pronounced spatiotemporal variations, particularly in the levels and distributions of nutrients such as carbon, nitrogen, and phosphorus. In spring, areas with high TOC and POC values were predominantly found in the nearshore and offshore regions near the Luanhe Estuary, as well as at offshore station S18 near the Dapuhe Estuary. This suggests a significant input of organic matter in these areas, likely originating from biological activities, such as the primary productivity of phytoplankton in river inlets or along the shoreline. While these organic matters provide essential nutrients for aquatic organisms, they may also lead to excessive organic loading in localized areas, thereby increasing the risk of eutrophication. The elevated TOC and POC levels at station S18 may be attributed to enhanced carbon sinks resulting from agricultural activities. In summer, high TOC and POC concentrations were primarily observed at nearshore station S17, indicating that organic carbon production or exogenous inputs were more concentrated in this area during the warmer months, potentially linked to phytoplankton production or human activities. Conversely, the elevated levels of DOC in both spring and summer suggest increased inputs of dissolved organic matter in specific areas, particularly near river mouths. These dissolved organic carbons can serve as nutrients for microorganisms, promoting their proliferation [25]. However, excessively high DOC concentrations may reduce water transparency, adversely affecting photosynthesis and disrupting the ecological balance of aquatic systems.

Eutrophication often results in excessive algal growth, which can lead to phytoplankton blooms and subsequent ecological issues such as hypoxia [26]. Changes in oxygen levels in the world’s oceans have been linked to the evolution of life and are considered one of the primary drivers of reduced biodiversity. Coastal eutrophication and hypoxia represent ongoing environmental crises, particularly in regions such as Asia, South America, and Africa, yet their specific drivers and mechanisms remain poorly understood [27]. The impact of anthropogenic ocean deoxygenation on marine life is garnering increasing attention, alongside concerns regarding ocean warming and acidification [3,28,29]. In this study area, the DO levels were found to be higher than those in Maowei Sea, the largest oyster culture bay in southwest China [30], and all stations recorded DO values exceeding the water quality standard of 5 mg/L, as specified by the Environmental Quality Standard for Surface Seawater of the People’s Republic of China. Therefore, it can be concluded that hypoxia is not present in this study area.

The distribution of nitrogen was primarily influenced by river inputs and human activities. The high TN, PN, and PON values observed in both spring and summer were concentrated in the northern region from Dai Estuary to Dapu Estuary, indicating significant impacts from river input. Nitrogen fertilizers and agricultural runoff are likely important sources of elevated nitrogen levels in these areas. In summer, the high PN and PON values extended to the vicinity of Yanghe Estuary (S20 and S21) and remained elevated near Luanhe (S4), suggesting that increased river runoff and agricultural activities during this season likely contributed more organic particulate matter to the marine environment. The relatively low TN levels in the Changli Reserve indicate that this area is more isolated, and effective management of the nature reserve may have mitigated the input of exogenous pollutants. In spring, the high TDN values were concentrated in the buffer zone and core area of the Changli Reserve at the Dabu River Estuary, likely related to the dissolved nitrogen inputs from river runoff, with agricultural and urban sewage being the primary sources. In summer, the high TDN values were concentrated at nearshore stations near the Yanghe and Luanhe Estuaries, closely associated with increased summer runoff, nitrogen fertilizer application, and sewage discharge, demonstrating clear exogenous input characteristics. The high nitrate concentrations in summer were primarily located south of the Dapu Estuary and along the vertical section of the Luanhe Estuary, indicating substantial impacts from agricultural runoff and sewage discharge. In agricultural runoff, nitrogen fertilizers are converted to nitrate through the nitrification processes. The area with elevated nitrate levels in spring was mainly situated in the offshore region between Xinkaikou and Luanhe Estuaries (S9), likely linked to the initial release of nitrogen fertilizers during the spring agricultural activities. The abnormal spike in nitrite levels at station S9 in spring may suggest active nitrification in this area, where ammonium is converted to nitrite, potentially due to insufficient dissolved oxygen in local waters, leading to nitrite accumulation. The distribution of high nitrite values in summer mirrored that of nitrate, indicating that both were similarly influenced by agricultural runoff and sewage discharge. However, the source of ammonium was more localized, primarily found around the periphery of the Changli Protected Area, and was not significantly affected by distance from the shore, possibly related to aquaculture activities outside the protected zone.

The input of ammonium from human activities represents a significant source of ammonium in coastal waters. In economically developed and densely populated regions such as Shenzhen Bay, China, 80% of the total nutrient load is attributed to human activities [22]. In winter, 81% of nitrates and 68% of ammonium in the Pearl River Estuary originate from river discharge [31]. In summary, agricultural runoff and sewage discharges are major sources of various nitrogen forms, including TN, TDN, nitrate, and nitrite. The sources of particulate nitrogen (PN and PON) are primarily derived from the production and decomposition of phytoplankton and other organisms, as well as organic particulate matter input from rivers. Seasonal variations are evident, with higher concentrations of PN and PON observed in summer due to increased biological activity.

Additionally, the areas with high phosphorus values in spring were predominantly found around offshore stations, whereas in summer, they were more concentrated around nearshore stations. This seasonal variation may be attributed to changes in temperature, precipitation, and human activities. Elevated phosphorus concentrations around coastal stations suggest that these water bodies may be influenced by exogenous phosphorus inputs, particularly from agricultural land and industrial emissions [25]. The high phosphorus concentrations around offshore stations may be linked to natural marine water circulation and sediment release processes [32]. In conclusion, the differences in the spatial distribution of nutrient salts reflect the influence of exogenous nutrient inputs across different regions. Coastal stations near estuaries receive more surface runoff and river input, resulting in significantly higher nutrient levels, indicating that these areas are more susceptible to human activities. In contrast, offshore stations are influenced by oceanic processes, hydrodynamic conditions, atmospheric deposition, and other factors, leading to high localized nutrient distributions.

4.2. Phosphorus, Nitrogen, and Carbon Element Cycling and Transformation Processes

The dynamics of carbon, nitrogen, and phosphorus levels in spring and summer are significantly influenced by seasonal environmental changes and external inputs, particularly temperature, precipitation, and anthropogenic activities. These factors lead to variations in the distribution of different forms of these elements across various spatiotemporal scales, reflecting the complex interactions within their respective cycling pathways. In terms of the carbon cycle, TOC and POC exhibit higher fluctuations and concentrations at different stations in spring, while showing smaller fluctuations in summer. The spatiotemporal distribution of DOC is generally more uniform. The increase in temperature during spring, coupled with enhanced photosynthesis and the active growth of primary producers such as phytoplankton, may contribute to elevated production of organic carbon, resulting in increased TOC and POC concentrations, particularly at stations S1 and S2. These stations are located near river inlet points and urban outfalls, which introduce substantial amounts of organic matter. In summer, high temperatures and strong sunlight stimulate more vigorous activities of microorganisms and phytoplankton. Although primary production capacity increases, the decomposition of organic matter also intensifies, leading to a more stable overall level of TOC and POC compared to spring. DOC, primarily composed of dissolved organic matter, tends to have a relatively stable production and decomposition process, resulting in minimal fluctuations across different seasons and stations. Regarding the nitrogen cycle, TN and TDN concentrations are higher in summer due to increased microbial activity associated with elevated temperatures. This includes enhanced ammonification and nitrification processes, which convert more organic nitrogen into dissolved inorganic nitrogen [33]. Additionally, increased precipitation during summer contributes to greater nitrogen input, particularly from agricultural and urban runoff, further promoting TDN concentration. PON, on the other hand, is primarily associated with plankton concentrations and organic detritus, which exhibit relatively minor seasonal changes and are less influenced by human activities, resulting in limited fluctuations across different stations and seasons. Some studies indicate that increased nitrogen nutrient inputs primarily enhance primary productivity, potentially decreasing microbial activity, increasing soil carbon sequestration capacity, and elevating nitrogen emissions. However, rising CO₂ levels can improve nitrogen fixation capacity and soil nitrogen availability [34]. Thus, in the context of global climate change, the carbon and nitrogen cycles are in a continuous state of dynamic adjustment [34,35].

For phosphorus, the elevated phosphorus concentrations observed in spring may result from phosphorus deposited during winter being released into the water column as temperatures rise in spring, particularly AP and TDP released from sediments [36]. Additionally, increased runoff during spring may introduce more phosphorus sources, especially at nearshore stations S1 and S2 [37]. Both AP and TDP are closely related to the dissolved state of phosphorus, and their variation trends are similar, indicating that their sources and transformation mechanisms are interconnected. TDP serves as one of the sources of AP, a relationship supported by the significant positive influence of TDP on AP observed in path analysis.

4.3. Adaptability of Coastal Bacterioplankton Communities

Studies conducted in the North Yellow Sea and Qinhuangdao Sea area have revealed the dominant microbial groups in the water are Proteobacteria, Bacteroidetes, Cyanobacteriota, Actinobacteria, and Planctomycetes [38]. These findings align with previous research that underscores the significant influence of environmental factors and nutrient elements on the species composition of microorganisms [39,40]. The microbial community in coastal waters of the Bohai Sea is particularly impacted by dissolved oxygen and nutrient salts, potentially due to the intrinsic characteristics of these microorganisms. Many microorganisms play crucial roles in natural processes such as nitrogen fixation and nitrification, making them highly susceptible to variations in nutrient availability. Furthermore, microbial metabolism is intrinsically linked to the presence of oxygen, thereby establishing a close relationship between the distribution of microbial communities and dissolved oxygen levels. Temperature, ammonium salt concentration, salinity, and dissolved oxygen have been identified as key factors influencing the composition and distribution of microbial communities. These factors suggest that the bacterial community structure undergoes seasonal shifts and variations in response to changes in nutrient salt concentration. Consequently, the functional potential of the microbial community fluctuates with seasonal and nutrient changes. These observations are consistent with previous studies examining the adaptive relationship between microbial community structure and environmental factors in various settings, including the global oceanic environment and simulated environments of wind erosion and sedimentation in grasslands [39,41]. This consistency reinforces the understanding that microbial communities are dynamic and responsive to their environmental contexts.

The results of this study indicate that Proteobacteria is a dominant microbial group across all samples, affirming its ubiquitous presence and significant role in various environments, including deep-water settings. This finding aligns with numerous studies on microbial diversity in seawater [42]. In the Proteobacteria found in seawater samples, the main subclasses are Gammaproteobacteria and Alphaproteobacteria. Gammaproteobacteria play a pivotal role in the marine sulfur cycle, particularly in the oxidation of hydrogen sulfide, which is a crucial aspect of sulfur cycling in marine habitats [43]. Conversely, Desulfobacterota are predominantly sulfate-reducing bacteria, involved in the reduction of sulfate [44,45]. The high abundance of both Gammaproteobacteria and Desulfobacterota observed in this study may be intimately linked to sulfur element transformations in the marine environment. Alphaproteobacteria are dominant in organic matter decomposition and are particularly adept at absorbing low-molecular-weight dissolved organic matter such as amino acids, proteins, glucose, and n-acetylglucosamine in diverse marine environments [46]. In the Changli Sea of the Bohai Sea, Alphaproteobacteria, especially the SAR11 clade, which is abundant, may be closely associated with organic matter decomposition. SAR11 bacteria are well adapted to oligotrophic conditions and are capable of oxidizing various methyl groups and single-carbon compounds [47]. In summary, the Proteobacteria found in this study, particularly Gammaproteobacteria and Alphaproteobacteria, exhibit distinct ecological functions related to sulfur cycling and organic matter decomposition, contributing to the nutrient cycle and overall ecosystem health in marine environments.

Furthermore, Bacteroidaceae emerges as another dominant group within the community structure, effectively consuming a substantial portion of high-molecular-weight dissolved organic matter in the ocean. This group specializes in decomposing polysaccharides and proteins [48]. Notably, Flavobacteriia, which belongs to the Bacteroidaceae family, exhibits high abundance and possesses the capability to decompose proteins and other polymeric-dissolved organic matter. An analysis conducted by Stevens et al. revealed that, in addition to Proteobacteria, Bacteroidetes also demonstrate high diversity and abundance in the Wadden Sea [49]. During summer, when sunlight is abundant and conditions are favorable, heterotrophic bacteria like Flavobacteriaceae can degrade organic matter derived from Cyanobacteriota and other algae, thereby obtaining energy for their growth. This explains their high abundance during this period [50]. Cyanobacteriota possesses characteristics of both plants and bacteria, containing chlorophyll-a, which enables them to carry out photosynthesis and produce oxygen. They can also absorb nutrients from surface waters for carbon fixation.

Planctomycetes bacteria participate in global nitrogen cycling and can convert ammonia to nitrogen via the anaerobic ammoxidation process in nutrition-restricted environments [51]. The Chloroflexi phylum plays a key role in the degradation of stubborn organic compounds such as cellulose at nearshore sites [52]. Large amounts of organic matter are imported from the land through estuaries, so some areas offshore are in a low-oxygen state. Acidobacteriota, SAR324, NB1-j, and other microorganisms are anaerobic or facultative anaerobic microorganisms and can grow well in this environment [53,54,55]. The large presence of these microbial communities highlights their importance in element cycling in estuarine environments.

Here, season is one key factor that dominates the spatial and temporal distribution of planktonic bacterial communities. A survey about the Zhoushan Sea shows that the distribution of bacterioplankton is affected by temperature and offshore distance [56]. Due to the large environmental changes in spring and summer, there are significant differences in their microbial communities. Spring is generally cooler, sunshine and rainfall gradually increase, water temperature rises, and microbial growth and metabolic activities accelerate. During the summer, increased rainfall increases surface runoff and, for these reasons, significantly increases the transport of nutrients to the ocean, which in turn stimulates the metabolic activity of planktic bacteria. This is also observed in the later stages of the bloom, when the SAR86 branch and OM43 group of Gamma-Proteobacteria become abundant [57]. These factors contributed to the frequent occurrence of summer algal blooms in this study, especially when nitrogen levels increased substantially. In this study, Verrucomicrobia in the west of Bohai Bay showed a significant positive correlation with salinity (p < 0.05). These results suggest that estuarine plankton bacterial communities may also be influenced by heavy metals, microplastics, and antibiotics imported through human activities [58,59,60]. In addition, future studies should focus more on revealing the combined effects of multiple factors.

Changes in marine microbial diversity can affect carbon sequestration by reducing the ability of marine ecosystems to buffer against climate change. When microbial diversity changes, the proportion of species able to adapt to a wider temperature range decreases, resulting in a diminished ability of microbial communities to adapt to temperature changes. This will not mitigate and may even exacerbate the increase in respiratory processes in marine ecosystems caused by warming, which releases more carbon dioxide into the atmosphere and reduces the function of marine ecosystems as carbon sequestration sites. Therefore, changes in microbial diversity have an important impact on carbon sequestration. Increasing microbial diversity may contribute to enhancing the carbon sequestration function of marine ecosystems, while loss of microbial diversity may weaken the ability of marine ecosystems to act as carbon sequestration sites. Therefore, in the process of coping with climate change and protecting biodiversity, the important role of microbial diversity should be fully considered.

The results of this microbial diversity study can provide key information for strategies to mitigate eutrophication in the Changli Gold Coast National Nature Reserve, mainly by aiding our understanding of microbial community structure and its changes, enabling us to assess nutritional status and formulate corresponding management measures. Microorganisms play an important role in marine ecosystems [12,13,14,15]. They not only provide most of the primary productivity but also participate in redox activities in the ocean, adjusting and promoting the formation and development of the dynamic balance of marine ecology [12,13,14,15]. This study of microbial diversity in the coastal waters of Changli can provide an in-depth understanding of the characteristics of microbial community structure, which is the basis for assessing the ecological health status of the sea area. Moreover, studying the temporal and spatial changes in microbial diversity can indirectly reflect the changes in seawater quality and nutrient content. For example, changes in the abundance of Planctomycetes bacteria and SAR324 [51] may be closely related to the levels of nutrients such as nitrogen and phosphorus, which are key factors in triggering eutrophication. Therefore, the results of this study can provide a scientific basis for judging whether there is a risk of eutrophication in the Changli Gold Coast National Nature Reserve.

5. Conclusions

This study provides a comprehensive view of the spatiotemporal distributions of elements (carbon, nitrogen, and phosphorus) and their influences on bacterioplankton communities in the coastal waters from Daihe Estuary to Luanhe Estuary in northern China. The results demonstrate significant temporal variability in the concentrations of various forms of carbon, nitrogen, and phosphorus during spring and summer, influenced by natural environmental factors and anthropogenic activities. In spring, increased biological activity, particularly phytoplankton growth, may lead to elevated levels of TOC and POC near river estuaries. Conversely, in summer, microbial decomposition processes likely contributed to the stabilization of carbon concentrations. Nitrogen concentrations were notably higher in summer, potentially attributed to enhanced microbial activity and nutrient inputs from agriculture and urban runoff. The phosphorus cycle was similarly affected by sediment release and runoff, with a significant correlation observed between TDP and AP.

Seasonal variations and human activities have a significant impact on the composition of bacterioplankton communities. During spring, the rise in temperatures and increase in precipitation boost the organic matter decomposition capabilities of certain microbes, such as Bacteroidetes. In contrast, during summer, higher temperatures, increased riverine inputs, and heightened tourism activities collectively enhance nutrient inputs, which stimulate the growth of photosynthetic microorganisms. Moreover, local environmental conditions play a crucial role in shaping the structure of bacterioplankton communities. Specifically, municipal wastewater discharge and coastal engineering activities artificially disrupt the natural nutrient balance in coastal areas.