养分资源脉冲供给对几种微藻种间竞争的影响

以淡水生态系统常见的5种微藻为研究对象,通过稳态条件下单一培养的方式获取各微藻在氮素或磷素缺乏条件下对应的生长特征参数和R*值,同时将5种微藻在养分资源脉冲供给的方式下进行混合培养,进而检测养分资源脉冲供给对几种微藻种间竞争的影响作用,并与基于稳态条件下所预测的微藻种间竞争结果进行比较。研究结果显示,在稳态条件下具有最小R*值的纤细角星鼓藻在与其它微藻的种间竞争过程中始终保持优势地位,而其余4种微藻在混合培养状态下的相对比重亦与其稳态条件下的R*值紧密相关。然而,资源脉冲供给条件下的竞争优势种纤细角星鼓藻能与其它种群数量较小的微藻共存。此外,在氮素和磷素的不同供给比例情况下,对应微藻群落的结构也会发生相应的变化。实验养分资源脉冲作用下多种微藻的共存现象与自然水体中的观测现象相一致,显示了资源脉冲可能是维持生物多样性水平的一个重要机制。

The effect of resource pulse supply on interspecific competition of a few algal species

Resource competition theory predicts that under equilibrium conditions, algal species with a minimal requirement for a specific limiting factor, or, in other words, algal species with the lowest R^* value for that limiting factor would become the superior competitor and assert its dominance. In contrast, algal species with higher R^* values would be out-competed by the superior competitor and thus undergo local extinction due to competitive exclusion. However, observations from natural aquatic ecosystems have repeated shown the fact that dominant algal species could coexist with algal species with small population size. Resource pulses of limiting nutrients might reduce competition intensity among algal species, and thus help solve this discrepancy between theoretical predictions and field observations. However, due to the existence of other confounding factors, such as regional species pool and spatial heterogeneity, our understanding of the effect of resource pulses on alleviating algal competition intensity and altering competitive outcome has been hindered. This study applied microcosms assembled with five algal species that are commonly found in natural freshwater ecosystem to test the effect of resource pulses on algal competitive outcome. Several growth parameters, including the half-saturation constant and the maximum specific growth rate, were determined using Monod model when individual algal species were grown as monocultures on a gradient of limiting nitrogen or phosphorous. Also, the R^* value of each algal species was calculated to predict the competitive outcome when algal species were grown as polycultures under non-equilibrium conditions when limiting nutrient resources were supplied as fluctuation forms. To reduce confounding effect, spatial heterogeneity effect was reduced by shaking microcosms frequently, and regional species pool effect was minimized by assembling all microcosms with same algal species with equal abundance initially. Therefore, the non-equilibrium conditions in studied microcosms are directly caused by resource pulse supply, and the effect of resource pulses on algal competitive outcome could be strictly tested. Also, the observed results under non-equilibrium conditions could be compared with theoretical predictions based on equilibrium state assumption, and the validity of resource competition theory would be further assessed. The results showed that one green algal species, Staurastrum gracile, had the lowest R^* value under either nitrogen- or phosphorous-limiting conditions. According to the resource competition theory under equilibrium settings, this algal species should be the superior competitor that would outcompete other algal species when grown together, ultimately resulting in the local extinction of other algal species due to competitive exclusion. In the present study, when resource pulses of limiting nutrients were supplied to simulate non-equilibrium conditions and when all five algal species were grown together, Staurastrum gracile remained the best competitor that was found in every microcosm with absolute dominance as predicted. However, this superior competitor coexisted with other algal species under resource pulse supply conditions. Clearly, resource pulses had lowered down algal competitive intensity and thus greatly reduced the possibility of competitive exclusion as long as resource pulse supply could persist. Additionally, for microcosms receiving different nitrogen to phosphorous ratios, corresponding community structure also varied in terms of species composition and identity. Our empirical result that multiple algal species could coexist with resource pulse supply is thus in line with many observational studies on natural aquatic ecosystem, indicating that resource pulse supply represents one of important mechanisms for the maintenance of biodiversity.