New Insights into How Nitrogen Run-off Affects Marine Sponges
Researchers from Te Herenga Waka—Victoria University of Wellington have taken a close look at how elevated nitrogen levels, primarily from agricultural run-off, could influence marine sponges. The study examined seven sponge species from two geographic regions—three species along Wellington’s coast in New Zealand and four from Lough Hyne, a protected marine reserve in County Cork, Ireland. The work, published in the Journal of Experimental Marine Biology and Ecology, seeks to understand whether nitrogen enrichment from farming practices impacts sponge health and, by extension, reef ecosystems that depend on these filter feeders.
Experiment Design: Simulating Run-off in the Lab
Lead author Gabriela Wood, a Ph.D. candidate at Te Herenga Waka, explains that the team simulated nitrogen-rich seawater at concentrations that could be reached after heavy rainfall events. The seven sponge species were exposed to a gradient of nitrogen levels, allowing researchers to observe short-term responses across diverse taxa. The goal was to gauge resilience and identify species that might be more sensitive to nutrient enrichment.
Key Findings: High Tolerance with Notable Exceptions
Overall, the sponges demonstrated strong tolerance to short-term increases in nitrogen, with survival rates exceeding 95% in most cases. This finding suggests a degree of resilience in sponge populations facing episodic nutrient pulses. However, not all species fared equally. One Irish species, Cliona celata, exhibited adverse effects at the highest nitrogen concentrations used in the experiments. Visible color changes and altered respiration rates indicated physiological stress in this sponge, signaling that elevated nutrients can push some species toward suboptimal functioning even if they do not immediately die from exposure.
Respiration as an Indicator of Stress
Respiration rate changes served as a crucial indicator of stress for several sponges. Two additional species from Lough Hyne also showed shifts in respiration under high nitrogen levels. While survival remained high during lab tests, the research team cautions that the laboratory setting cannot capture all the complexities of the natural environment, where secondary effects from nutrient increases may unfold over longer timescales.
Environmental Implications: Beyond Direct Toxicity
Professor James Bell, a marine biologist at Te Herenga Waka and co-author of the study, notes that the story is not simply about nitrogen poisoning. Elevated nitrogen can fuel plankton blooms, which can in turn disrupt food webs, alter light penetration, and change the balance of marine communities. Sponges depend on clean, nutrient-balanced waters to thrive, and blooms can create cascading impacts that hinder their growth and survival, especially for juvenile sponges and delicate species.
What This Means for Conservation and Future Research
The study highlights both the resilience and vulnerabilities within sponge communities. While many species showed short-term tolerance to nitrogen, the long-term effects remain uncertain. The researchers emphasize the need for ongoing monitoring to track chronic exposure, as well as studies that explore how other sponge species respond over extended periods and under varying environmental conditions such as temperature and salinity fluctuations.
Takeaway: Balancing Agricultural Practices and Marine Health
The findings underscore a broader environmental message: nutrient run-off from agriculture can impact coastal ecosystems in nuanced ways that extend beyond immediate mortality. By improving land-management practices, reducing fertilizer application in sensitive coastal zones, and maintaining robust watershed protection, communities can support sponge diversity and the services these animals provide, from cleaning seawater to supporting biodiversity and reef structure.
About the Research
The work brings together sponge specimens from New Zealand and Ireland, reflecting the global relevance of nutrient management in coastal regions. While the results suggest a general tolerance to nitrogen run-off in the tested species, they also remind us that ecosystem responses are species-specific and context-dependent. The team calls for long-term field studies to verify lab findings and to identify which species are most at risk under ongoing nutrient enrichment.
