N vs P effects on seagrass meadows

We examined the spatial extent of nitrogen (N) and phosphorus (P) limitation of each of the major benthic primary producer groups in Florida Bay (seagrass, epiphytes, macroalgae, and benthic microalgae) and characterized the shifts in primary producer community composition following nutrient enrichment. We established 24 permanent 0.25-m2 study plots at each of six sites across. Florida Bay and added N and P to the sediments in a factorial design for 18 mo. Tissue nutrient content of the turtlegrassThalassia testudinum revealed a spatial pattern in P limitation, from severe limitation in the eastern bay (N:P>96:1), moderate limitation in two intermediate sites (approximately 63:1), and balanced with N availability in the western bay (approximately 31:1). P addition increasedT. testudinum cover by 50–75% and short-shoot productivity by up to 100%, but only at the severely P-limited sites. At sites with an ambient N:P ratio suggesting moderate P limitation, few seagrass responses to nutrients occurred. Where ambientT. testudinum tissue N:P ratios indicated N and P availability was balanced, seagrass was not affected by nutrient addition but was strongly influenced by disturbance (currents, erosion). Macroalgal and epiphytic and benthic microalgal biomass were variable between sites and treatments. In general, there was no algal overgrowth of the seagrass in enriched conditions, possibly due to the strength of seasonal influences on algal biomass or regulation by grazers., N addition had little effect on any benthic primary producers throughout the bay. The Florida Bay benthic primary producer community was P limited, but P-induced alterations of community structure were not uniform among primary producers or across Florida Bay and did not always agree with expected patterns of nutrient limitation based on stoichiometric predictions from field assays ofT. testudinum tissue, N:P ratios.

本研究针对佛罗里达湾（Florida Bay）内各主要底栖初级生产者（benthic primary producer）类群——海草（seagrass）、附生藻类（epiphytes）、大型藻类（macroalgae）及底栖微藻（benthic microalgae）——的氮（N）、磷（P）限制空间范围展开调查，并解析了营养盐富集后初级生产者群落组成的变化规律。我们在佛罗里达湾的6个站位各布设24个永久性0.25 m²样方，采用析因设计（factorial design）向沉积物中添加氮、磷，实验持续18个月。海龟草（Thalassia testudinum）的组织营养盐含量显示出磷限制的空间格局：湾东部为严重磷限制（氮磷比>96:1），两个中间站位为中度磷限制（约63:1），湾西部则氮磷供应平衡（约31:1）。仅在严重磷限制的站位，添加磷可使海龟草盖度提升50%~75%，短枝生产力最高提升100%。在环境氮磷比提示中度磷限制的站位，海草对营养盐添加几乎无响应。当海龟草组织氮磷比显示氮磷供应平衡时，海草不受营养盐添加影响，但会受到扰动（水流、侵蚀）的强烈调控。大型藻类、附生藻类及底栖微藻的生物量在不同站位与处理组间存在差异。整体而言，富营养条件下未出现藻类覆盖海草的现象，这可能与季节因素对藻类生物量的显著影响或牧食者（grazers）的调控作用有关；全湾范围内添加氮对所有底栖初级生产者均无明显影响。佛罗里达湾底栖初级生产者群落整体受磷限制，但磷添加引发的群落结构变化在不同初级生产者类群间及全湾范围内并不一致，且并不总是与基于海龟草组织氮磷比野外测定结果的化学计量学预测的营养盐限制预期模式相符。