<p>Edible macroalgal cultivation is increasingly promoted as a nature-based solution to mitigate coastal eutrophication and improve seawater quality. However, the species-specific impacts and spatial extent of these ecological effects remain poorly understood, particularly in semi-enclosed bays with complex hydrodynamics. This study aims to quantify the biogeochemical influence of two widely cultivated species—<i>Porphyra haitanensis</i> and <i>Hizikia fusiformis</i>—on seawater carbonate chemistry and nutrient levels in Yueqing Bay, eastern China. High-resolution field surveys were conducted at 52 stations, enabling direct comparisons between cultivated and non-cultivated waters. Geostatistical modeling, including spherical semivariograms and Empirical Bayesian Kriging, was applied to delineate species-specific influence zones and quantify changes in key water quality parameters. <i>P. haitanensis</i> farming induced broad, kilometer-scale improvements in seawater chemistry, including elevated dissolved oxygen (DO) (+ 2.72%) and pH (+ 0.09 units), and significantly lower partial pressure of CO₂ (pCO₂) (− 118 µatm), relative to distant reference sites (all <i>p</i> &lt; 0.05). A slight increase in total phosphorus (TP) (+ 0.007 mg L^− 1) was also observed, likely reflecting nearby riverine inputs. In contrast, <i>H. fusiformis</i> cultivation produced more localized (&lt; 100&#xa0;m) but significant changes, including reductions in dissolved inorganic carbon (DIC) (− 1.84 mg L^− 1) and pCO₂ (− 82.6 µatm), alongside increases in DO (+ 1.72%), pH (+ 0.02 units), and chlorophyll-<i>a</i> (Chl-<i>a</i>) (+ 0.72&#xa0;µg L^− 1) (all <i>p</i> &lt; 0.05). These results provide the first fine-scale, species-resolved spatial assessment of macroalgal farming effects on water quality in a semi-enclosed bay. By quantifying distance-dependent ecological responses, this study offers science-based guidance for spatial planning, nutrient management, and blue carbon integration—particularly as the routine harvest of biomass facilitates net carbon export from coastal waters. These findings highlight the potential of macroalgal farming as a scalable, multifunctional nature-based solution for sustainable aquaculture and climate mitigation.</p> Graphical Abstract <p>Based on the graphical snapshot, this study was conducted to evaluate species-specific effects of seaweed farming on coastal seawater chemistry and to delineate the spatial extent of these ecological benefits. High-resolution field surveys at 52 stations in Yueqing Bay, Wenzhou, Zhejiang Province, China, measured pH, dissolved oxygen (DO), partial pressure of CO₂ (pCO₂), nutrients, and chlorophyll-<i>a</i> (Chl-<i>a</i>) to capture gradients from farm interiors to offshore reference sites. Sampling was designed to systematically track changes in relation to distance from cultivation zones, enabling robust characterization of the environmental footprint of <i>Porphyra haitanensis</i> and <i>Hizikia fusiformis</i> farms. Spherical semivariogram modeling and Empirical Bayesian Kriging interpolation were applied to generate detailed spatial maps of seawater parameters. Results revealed that <i>P. haitanensis</i> cultivation produced broad (&gt; 3.9&#xa0;km) increases in pH and DO, along with reductions in pCO₂, whereas <i>H. fusiformis</i> showed strong but highly localized (&lt; 100&#xa0;m) reductions in pCO₂ and dissolved inorganic carbon (DIC), accompanied by elevated Chl-<i>a</i>. These findings deliver high-resolution evidence of species-specific, distance-dependent water quality improvements resulting from macroalgae cultivation, providing practical guidance for aquaculture planning, targeted nutrient management, and sustainable coastal management strategies.</p>